NREL is a national laboratory of the U.S. Department of Energy

Office of Energy Efficiency & Renewable Energy

Operated by the Alliance for Sustainable Energy, LLC

This report is available at no cost from the National Renewable Energy

Laboratory (NREL) at www.nrel.gov/publications.

Contract No. DE-AC36-08GO28308

Foothill Transit Battery Electric 

Bus Demonstration Results

Leslie Eudy, Robert Prohaska, Kenneth Kelly, 

and Matthew Post

National Renewable Energy Laboratory

Technical Report

NREL/TP-5400-65274

January 2016
NREL is a national laboratory of the U.S. Department of Energy

Office of Energy Efficiency & Renewable Energy

Operated by the Alliance for Sustainable Energy, LLC

This report is available at no cost from the National Renewable Energy

Laboratory (NREL) at www.nrel.gov/publications.

Contract No. DE-AC36-08GO28308

National Renewable Energy Laboratory

15013 Denver West Parkway

Golden, CO 80401

303-275-3000 • www.nrel.gov

Foothill Transit Battery Electric 

Bus Demonstration Results

Leslie Eudy, Robert Prohaska, Kenneth Kelly, 

and Matthew Post

National Renewable Energy Laboratory

Prepared under Task No. WW4K.1000

Technical Report

NREL/TP-5400-65274

January 2016
NOTICE

This report was prepared as an account of work sponsored by an agency of the United States government. 

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express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of 

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or favoring by the United States government or any agency thereof. The views and opinions of authors expressed 

herein do not necessarily state or reflect those of the United States government or any agency thereof.

This report is available at no cost from the National Renewable Energy

Laboratory (NREL) at www.nrel.gov/publications.

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Cover Photos by Dennis Schroeder: (left to right) NREL 26173, NREL 18302, NREL 19758, NREL 29642, NREL 19795. Bus Photo by Leslie 

Eudy, NREL.

NREL prints on paper that contains recycled content.
iii

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Acknowledgments

This evaluation at Foothill Transit would not have been possible without the support and 

cooperation of many people. The authors thank the following individuals:

California Air Resources Board

Yachun Chow

Jennifer Lee

Craig Duehring

U.S. Department of Energy’s National Renewable Energy Laboratory

Keith Wipke

Jen Kurtz

Foothill Transit

Roland Cordero

TJ Nass

Andrew Papson

Proterra

Mike Finnern

Unless otherwise noted, all photos by L. Eudy, NREL
iv

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Acronyms and Abbreviations

Ah amp-hours

BEB battery electric bus

CARB California Air Resources Board

CNG compressed natural gas

DGE diesel gallon equivalent

DOE U.S. Department of Energy

ESS energy storage system

FCEB fuel cell electric bus

ft feet

FTA Federal Transit Administration

GGE gasoline gallon equivalent

GVWR gross vehicle weight rating

hp horsepower

HVAC heating, ventilation, and air conditioning

in. inches

kg kilograms

kW kilowatts

kWh kilowatt hours

lb pounds

MBRC miles between roadcalls

mph miles per hour

NREL National Renewable Energy Laboratory

PMI preventive maintenance inspection

psi pounds per square inch

PTC Pomona Transit Center

SI International System of Units

SOC state of charge

TIGGER Transit Investments for Greenhouse Gas and Energy 

Reduction

TRL technology readiness level

ZBus zero emission bus
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Definition of Terms

Availability: The number of days the buses are actually available compared to the days that the 

buses are planned for operation expressed as percent availability.

Clean point: For each evaluation, NREL works with the project partners to determine a starting 

point—or clean point—for the data analysis period. The clean point is chosen to avoid some of 

the early and expected operations problems with a new vehicle going into service, such as early 

maintenance campaigns. In some cases, reaching the clean point may require 3 to 6 months of 

operation before the evaluation can start.

Deadhead: The miles and hours that a vehicle travels when out of revenue service with no 

expectation of carrying revenue passengers. Deadhead includes leaving or returning to the garage 

or yard facility and changing routes.

Miles between roadcalls (MBRC): A measure of reliability calculated by dividing the number of 

miles traveled by the number of roadcalls. (Also known as mean distance between failures.) 

MBRC results in the report are categorized as follows: 

• Bus MBRC: Includes all chargeable roadcalls. Includes propulsion-related issues as well 

as problems with bus-related systems such as brakes, suspension, steering, windows, 

doors, and tires.

• Propulsion-related MBRC: Includes roadcalls that are attributed to the propulsion system. 

Propulsion-related roadcalls can be caused by issues with the transmission, batteries, and 

electric drive.

• Energy storage system (ESS)-related MBRC: Includes roadcalls attributed to the energy 

storage system only.

Revenue service: The time when a vehicle is available to the general public with an expectation 

of carrying fare-paying passengers. Vehicles operated in a fare-free service are also considered 

revenue service.

Roadcall: A failure of an in-service bus that causes the bus to be replaced on route or causes a 

significant delay in schedule. The analysis includes chargeable roadcalls that affect the operation 

of the bus or may cause a safety hazard. Non-chargeable roadcalls can be passenger incidents 

that require the bus to be cleaned before going back into service, or problems with an accessory 

such as a farebox or radio.
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Executive Summary

In October 2010, Foothill Transit began a demonstration of three Proterra battery electric buses

(BEBs) in its service area located in the San Gabriel and Pomona Valley region of Los Angeles 

County, California. The agency had a goal of evaluating the technology to determine if it could 

meet service requirements and was feasible for selected Foothill routes. The demonstration went 

well, and Foothill moved forward with an order of 12 next-generation BEBs. In March 2014, 

Foothill Transit began operating the new fleet in its service area. These electric buses, produced 

by Proterra, are 35-foot, composite body buses that are capable of being charged quickly on 

route. 

Foothill Transit is collaborating with the California Air Resources Board (CARB) and the U.S. 

Department of Energy’s (DOE’s) National Renewable Energy Laboratory (NREL) to evaluate 

the buses in revenue service. NREL has been evaluating advanced technology buses under 

funding from DOE and the U.S. Department of Transportation’s Federal Transit Administration 

(FTA). The objectives of these evaluations are to provide comprehensive, unbiased evaluation 

results of advanced technology bus development and performance compared to conventional 

baseline vehicles.

CARB staff has been gathering data on zero-emission buses (ZBuses) to assess the status of the 

technology. The majority of ZBus data collected and reported to date are for fuel cell electric 

buses. CARB would like to have similar analysis and reporting for the other primary ZBus 

technology being adopted in the state of California, that is, BEBs. CARB has enlisted NREL to 

conduct a third-party evaluation of the Foothill Transit fleet.

In 2014, Foothill purchased 12 BEBs from Proterra through a $10.2 million grant under FTA’s 

Transit Investments for Greenhouse Gas and Energy Reduction (TIGGER) Program. Foothill’s 

project goal was to fully electrify one route in its service area—Line 291—and to investigate the 

feasibility of the technology for other routes. The buses are charged on route at a charging station 

built at a transit center mid-way along the route. 

The focus of this evaluation is to compare performance of the BEBs to that of conventional 

technology and to track progress over time toward meeting performance targets. In the 

commercialization process that begins at technology readiness level (TRL) 1—basic 

research/concept—and ends at TRL 9—commercial deployment—NREL considers the BEBs to 

be at TRL 7. At this point of development, the manufacturers’ goals for the demonstration are to 

verify that the BEB performance meets the technical targets and identify any issues that need to 

be resolved. Selecting a comparable baseline bus for a project can be challenging, especially 

when evaluating a unique design such as the Proterra BEB. Foothill does not operate 

conventional buses that are similar in size, weight, and year to the BEBs, so an apples-to-apples 

comparison is not possible for some performance metrics. The primary baseline buses selected 

are eight new NABI compressed natural gas (CNG) buses of the same model year as the BEBs. 

Like the BEBs, the NABI CNG buses are under warranty and should have very low maintenance 

costs.

This report provides data on the buses from April 2014 through July 2015. The data period for 

the eight new NABI CNG buses used as a baseline comparison begins in October 2014 when the 
vii

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buses were placed into service. Table ES-1 provides a summary of results for several categories 

of data presented in this report. From the start of the demonstration through July 2015, the BEBs

have traveled more than 401,000 miles. This equates to an average of 33,437 miles per BEB

during the data period.

Table ES-1. Summary of Evaluation Results

Data Item BEB CNG

Number of buses 12 8

Data period 4/14–7/15 10/14–7/15

Number of months 16 10

Total mileage in period 401,244 364,373

Average total miles per bus 33,437 45,547

Average monthly mileage per bus 2,333 4,555

Total operating hours 47,462 —

Availability (85% is target) 90 94

Fuel economy (kWh/mile or miles/GGEa) 2.15 4.04

Fuel economy (miles/DGEb) 17.48 4.51

Average speed (mph) 10.6 17.6

Miles between roadcalls (MBRC) – bus 9,331 45,547

MBRC – propulsion system only 25,078 91,093

MBRC – ESSc only 133,748 —

Total maintenance ($/mile)d $0.16 $0.18

Maintenance – propulsion system only ($/mile) $0.02 $0.08 a Gasoline gallon equivalent.

b Diesel gallon equivalent.

c Energy storage system. d Work order maintenance cost.

The average monthly operating mileage for the BEBs for the evaluation period is 2,333 miles, 

which is about half that of the CNG buses. This is expected, considering that the BEBs are 

operated primarily on Line 291 and the CNG buses are randomly dispatched on all routes out of 

the Pomona Depot, including express and commuter routes, which have much higher average 

speeds. The results presented in this report are based on the planned route for the buses and do 

not indicate a specific limitation of the technology.

During the evaluation period, the average availability for the BEBs was 90% compared to 94% 

for the CNG baseline buses. The per-bus availability for the BEBs ranged from a high of 98% to 

a low of 62%. One specific bus experienced several issues that kept it out of service for extended 

periods of time. The majority of issues were for general bus problems—repair of accident 

damage and the air conditioning system—and not due to any advanced technology component. 

None of the other buses had an availability of less than 85%, which indicates that this bus could 

be considered an outlier. If this bus is removed from the data set, the overall fleet availability is 

93%.

The BEBs had an overall average efficiency of 2.15 kWh per mile, which equates to 17.48 miles 

per diesel gallon equivalent (DGE). The CNG buses had an average fuel economy of 4.04 miles 

per gasoline gallon equivalent (GGE), which equates to 4.51 miles per DGE. These results 

indicate that the BEBs have an average fuel economy that is nearly 4 times higher than that of 

the CNG buses.
viii

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Reliability, measured as miles between roadcalls (MBRC), was high during the data period. The 

overall bus MBRC for the BEB fleet was more than 9,000. During the data period, there were 

only three roadcalls that were attributed to issues with the ESS, the primary power system for the 

buses. This resulted in an ESS-related MBRC of more than 133,000. This is exceptional for an 

advanced technology bus in the early stage of commercialization. The CNG baseline buses 

achieved an overall bus MBRC that was more than 45,000, which is expected of a new, fully 

commercial product.

In addition to analyzing the BEB performance, NREL provides a cost analysis and comparison.

Maintenance costs for both the BEBs and CNG buses were low because they are under warranty. 

During the reporting period, the BEBs had a maintenance cost per mile that was slightly lower 

(11% less) than that of the CNG buses.

This project has also provided an opportunity for DOE to conduct a detailed evaluation of the 

BEBs and charging infrastructure. Under funding from DOE’s Vehicle Technologies Office, 

NREL’s Fleet Test and Evaluation team is collecting detailed data from the Foothill BEBs to 

assess vehicle and component performance and conduct detailed drive cycle analysis, as well as 

to build and refine vehicle models for further analysis. Proterra provided in-use raw vehicle data 

for this analysis including details on vehicle speed; GPS location; battery pack state of charge 

(SOC); battery pack current; battery pack voltage; inverter current; inverter voltage; motor 

heating, ventilating, and air conditioning (HVAC) status; and more. Additional analysis is still 

required to draw significant conclusions on the technology, but the findings to date include the 

following:

• The average energy efficiency is 2.15 kWh per mile over 399,663 miles of use.

• The average battery pack SOC is 75.4%, indicating a possibility for a usage window 

shift.

• The average runtime per day is 13.2 hours with an average of 13 charges per day. Each 

charge averages 20 kWh energy delivered.

• Accessory loads contribute to the overall range capability, as more than 50% of “system 

on” time is spent at a speed of 0 mph where lighting and HVAC loads are still required. 

Foothill and Proterra report that the project went extremely well because they worked carefully 

up front to plan and execute the project. There have been many achievements for the 

demonstration, including:

• The project deployed 12 fast charge buses to fully electrify one route.

• The BEB fleet operated well, with no major issues due to the advanced-technology 

components. Bus-related components were the cause of the majority of issues.

• The on-route fast chargers operated reliably with minimal issues, none of which resulted 

in downtime for the buses.

Advanced technology demonstrations typically experience challenges and issues that need to be 

resolved. The challenges and lessons learned from the demonstration included bus-related 

problems as well as programmatic issues. One major challenge is addressing demand charges 
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and time of use charges that affect electricity cost. This will be a major challenge for any fleet 

looking to deploy electric buses that charge during peak times. The industry needs to work on a 

permanent solution for all BEB adopters to keep costs reasonable in the future. Another 

challenge is training operators and maintenance staff in the differences between BEBs and 

conventional buses. Foothill reports that there is still a lot of learning as the agency ramps up to a 

larger BEB fleet. The team needs to understand how service can transition to a higher number of 

buses.
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Table of Contents

Introduction ................................................................................................................................................. 1

ZBus Demonstrations in California........................................................................................................ 2

Fleet Profile—Foothill Transit Agency.................................................................................................. 2

Manufacturer Profile—Proterra.............................................................................................................. 3

Foothill Transit ZBus Project................................................................................................................. 4

Advanced Technology Bus Development Process—Technology Readiness Levels.......................... 6

Bus Technology Descriptions ................................................................................................................... 8

Charging and Maintenance Facilities...................................................................................................... 11

Fast-Charge Station .............................................................................................................................. 11

In-Service Operations Evaluation Results.............................................................................................. 14

Route Assignments............................................................................................................................... 14

Bus Use and Availability...................................................................................................................... 14

Energy Use, Fuel Economy, and Cost.................................................................................................. 18

Roadcall Analysis................................................................................................................................. 22

Maintenance Analysis .......................................................................................................................... 23

Total Work Order Maintenance Costs........................................................................................... 23

Work Order Maintenance Costs Categorized by System............................................................... 26

Propulsion-Related Work Order Maintenance Costs..................................................................... 27

Detailed Technology Evaluation Results................................................................................................ 29

Detailed Data Collection Approach...................................................................................................... 29

Results.................................................................................................................................................. 30

Detailed Analysis Summary Findings.................................................................................................. 35

Summary of Achievements and Challenges .......................................................................................... 36

Lessons Learned................................................................................................................................... 37

What’s Next................................................................................................................................................ 39

Future Analysis..................................................................................................................................... 39

Contacts..................................................................................................................................................... 40

References and Related Reports............................................................................................................. 41

Appendix A: TRL Guideline Table ........................................................................................................... 42

Appendix B: Foothill Fleet Summary Statistics..................................................................................... 43

Appendix C: Fleet Summary Statistics—SI Units.................................................................................. 49
1

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Introduction

In October 2010, Foothill Transit began a demonstration of three Proterra battery electric buses

(BEBs) in its service area located in the San Gabriel and Pomona Valley region of Los Angeles 

County, California. The agency had a goal of evaluating the technology to determine if it could 

meet service requirements and was feasible for selected Foothill routes. The demonstration went 

well, and Foothill moved forward with an order of 12 next-generation BEBs. In March 2014, 

Foothill Transit began operating the new fleet in its service area. These electric buses, produced 

by Proterra, are 35-foot, composite body buses that are capable of being charged quickly on 

route.

Foothill Transit is collaborating with the California Air Resources Board (CARB) and the U.S. 

Department of Energy’s (DOE’s) National Renewable Energy Laboratory (NREL) to evaluate 

the buses in revenue service. NREL has been evaluating advanced technology buses under 

funding from DOE and the U.S. Department of Transportation’s Federal Transit Administration 

(FTA). The focus of these evaluations over the last 10 years has been fuel cell electric buses 

(FCEBs). NREL uses a standard data-collection and analysis protocol originally developed for 

DOE heavy-duty vehicle evaluations. The objectives of these evaluations are to provide 

comprehensive, unbiased evaluation results of advanced technology bus development and 

performance compared to conventional baseline vehicles.

CARB, primarily through evaluations conducted by NREL, has been monitoring the 

development progress of FCEBs being demonstrated in California and other parts of the United 

States. More recently, transit agencies have begun to demonstrate BEBs. The introduction of 

opportunity fast charging has addressed the early range issues that were a challenge for 

deployment of BEBs. CARB staff has been gathering data on zero-emission buses (ZBuses) to 

assess the status of the technologies. The majority of ZBus data collected and reported to date are 

for FCEBs. CARB would like to have similar analysis and reporting for the other primary ZBus 

technology being adopted in the state of California, that is, BEBs. CARB has enlisted NREL to 

conduct a third-party evaluation of the Foothill Transit fleet.

This project has also provided an opportunity for DOE to conduct a detailed evaluation of the 

BEBs and charging infrastructure. Under funding from DOE’s Vehicle Technologies Office, 

NREL’s Fleet Test and Evaluation team has evaluated electric and plug-in electric vehicles in a 

variety of medium- and heavy-duty applications. These evaluations involve collection of raw 

second-by-second data from the vehicles in service, including GPS location; battery pack state of 

charge (SOC); battery pack current; battery pack voltage; inverter current; inverter voltage;

motor heating, ventilating, and air conditioning (HVAC) status; and more. The Fleet Test and 

Evaluation team is collecting these data from the Foothill BEBs to assess vehicle and component 

performance and conduct detailed drive cycle analysis, as well as to build and refine vehicle 

models for further analysis.

This report provides data on the buses from April 2014 through July 2015. Data are provided on 

a selection of compressed natural gas (CNG) buses as a baseline comparison. The early results of 

the detailed drive cycle and component analysis are summarized in the section “Detailed 

Technology Evaluation Results.” The team is still collecting data on the BEBs and charging 

equipment and plans to publish a report outlining all of the analysis results in the future. 
2

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ZBus Demonstrations in California

CARB’s 2000 “Fleet Rule for Transit Agencies”1 has been the primary driver for demonstrations 

of advanced technology buses in the state of California. This rule set more stringent emission 

standards for new urban bus engines and promoted advances in the cleanest technologies, 

specifically ZBuses. The fleet rule required transit agencies to choose a compliance path—

alternative fuel or diesel—for meeting emission standards. This selection determined the fuel 

type for new bus acquisitions through model year 2015. The alternative fuel path could include 

low-emission alternative fuels such as compressed or liquefied natural gas, propane, electricity, 

hydrogen, or another advanced technology (such as gasoline hybrid-electric). Agencies choosing 

the diesel path were required to reduce fleet average emissions through methods such as 

purchasing the cleanest diesel engines and retrofitting existing diesel engines with emission 

control devices (i.e., diesel particulate filters).

Under the rule, agencies with more than 200 buses must include ZBuses as 15% of new bus 

purchases. Agencies choosing the diesel path were scheduled to meet these requirements on a 

more accelerated timeline than those on the alternative fuel path. Diesel path fleets were also 

required to demonstrate the use of ZBus technology in revenue service. ZBus technologies that 

qualify for this regulation include electric propulsion (battery or trolley buses) and fuel cell 

propulsion. At the time of the original ruling, 11 agencies in the state had more than 200 buses. 

Of those agencies, five had selected the diesel path and were subject to the required ZBus 

demonstration. Those agencies selected fuel cell bus technology over BEBs, primarily because 

the state of electric bus technology at the time required overnight charging for a very limited 

range. 

Since that time, development of higher-energy-capacity traction battery designs has improved 

significantly with the introduction of lithium-based batteries. The improved performance that 

lithium-based batteries provide resulted in a renewed interest in developing battery-only 

propulsion for full-sized transit buses. This new approach to battery-only propulsion includes 

opportunity charging of the batteries while the bus is out on route, usually at layover points in the 

route design. This on-route, fast-charge capability addresses many of the concerns over lower 

range and long charge times for BEBs. As a result, a number of transit agencies in California 

(and other areas of the United States) began purchasing small fleets of BEBs for demonstrations.

Fleet Profile—Foothill Transit Agency

Foothill Transit serves a 327-square-mile area covering the San Gabriel and Pomona Valley 

region of Los Angeles County. Foothill’s administrative office is located in West Covina, 

California. Foothill Transit is governed by a Joint Powers Authority of 22 member cities and the 

County of Los Angeles with representation from the following areas:

Arcadia Azusa

Baldwin Park Bradbury

Claremont Covina

 1 Fact Sheet: Fleet Rule for Transit Agencies: Urban Bus Requirements, California Air Resources Board, 

http://www.arb.ca.gov/msprog/bus/ub/ubfactsheet.pdf.
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Diamond Bar Duarte

El Monte Glendora

Industry Irwindale

La Puente La Verne

Monrovia Pasadena

Pomona San Dimas

South El Monte Temple City

Walnut West Covina

Los Angeles County

Foothill operates 36 local and express routes including commuter runs to downtown Los 

Angeles. The current bus fleet consists of 315 CNG buses and 15 BEBs. Figure 1 shows the 

Foothill Transit service area.

Figure 1. Foothill Transit service area

The agency began a path to cleaner buses in 2002 by adding CNG buses to its fleet. The agency 

retired its last diesel bus in 2013, making the fleet 100% alternative fuel. Foothill Transit’s 

commitment to clean and efficient technologies has led the agency to initiate additional projects 

such as adding photovoltaic panels and making efficiency improvements to its facilities, 

installing a water-saving bus wash, and purchasing zero-emission BEBs.

Manufacturer Profile—Proterra

Proterra was founded in 2004 in Golden, Colorado, with a goal of producing advanced 

technology heavy-duty vehicles that emit zero emissions and are domestically fueled. In 2006, 

Proterra was awarded a grant through FTA’s National Fuel Cell Bus Program to design, build, 
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and demonstrate a battery-dominant hybrid fuel cell bus. The company elected to use a 35-foot

composite body to keep the weight down and maximize efficiency. Proterra’s propulsion system 

was based on an all-electric design with the capability to add a fuel cell power system as a range 

extender. Taking its all-electric concept one step further, Proterra added the capability for fast

charging while on route. The capability to charge on route addressed the range issue, which was 

the biggest challenge for deployment of BEBs at the time. The National Fuel Cell Bus Program

award helped Proterra acquire the start-up capital to begin manufacturing this electric bus.

Foothill Transit was Proterra’s first customer for its BEB design—the EcoRide. Foothill’s threebus order was produced in Proterra’s Colorado facility. As orders for the BEB increased, Proterra 

set up an assembly line in a new facility in South Carolina to produce the bus more 

economically. In 2011, the company moved its entire operation to the Greenville, South 

Carolina, facility. Since that time, Proterra has produced 59 of its 35-foot EcoRide buses, which 

are in service in nine fleets in the United States.

Proterra’s next-generation design is a 40-foot, composite body electric bus called the Catalyst. 

This bus is available with a fast-charge energy storage system (ESS) or an extended range, indepot charging system. Foothill will be the first customer for the extended range version of this 

design with an order of 13 buses. Foothill Transit has also ordered two Catalyst short range fastcharge models. As of September 2015, Proterra has orders for 50 of the Catalyst buses, which 

will bring the total number of Proterra BEBs in the United States to 109. Options on the current 

contracts add a potential 300 more Proterra BEBs to the overall number. Proterra is in the 

process of expanding and has added an office in the San Francisco Bay Area. The company was 

awarded a grant by the California Energy Commission in April 2015 to expand its manufacturing 

capability and is planning a second facility in the City of Industry, California.

Foothill Transit ZBus Project

Foothill Transit’s first three BEBs—funded through an American Recovery and Reinvestment 

Act grant—were delivered in 2011 and were placed in service on Line 291. The demonstration 

team reviewed the different routes and selected Line 291 as the optimal route for the technology. 

Line 291 is a 16.1-mile route that travels between La Verne and Pomona with minimal deadhead 

distance from the Pomona depot. The line is heavily traveled and serves a transit dependent 

community. The route loops through the Pomona Transit Center (PTC) in both directions, 

making it an ideal location for the fast charger system. Over the next year, the agency worked 

with Proterra to evaluate the technology, determine if it could meet service requirements, and 

investigate the feasibility for other Foothill routes. The demonstration went well, and Proterra 

used the lessons learned by the team to make upgrades to the system. After this initial testing, 

Foothill moved forward with plans to procure additional BEBs.

In 2013, Foothill was awarded a $10.2 million grant through the second round of FTA’s Transit 

Investments for Greenhouse Gas and Energy Reduction (TIGGER) Program to acquire nine

more fast-charge BEBs. The TIGGER Program made funds available for capital investments 

over a 3-year period from 2009 through 2011 that would reduce greenhouse gas emissions and/or 

lower the energy use of public transportation systems. Foothill replaced older CNG buses that 

had reached the end of useful life.
5

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The Foothill TIGGER project goal was to fully electrify Line 291, which requires seven buses 

during peak hours. The agency adjusted the schedule to accommodate time for charging the 

buses. The schedule already included some layover time, so the primary adjustment was to allow 

for the 45 seconds it takes to dock the bus. Figure 2 shows the route map for Line 291. The 

additional buses would be used as spares to allow for maintenance downtime, and also as fill-in 

buses for other appropriate routes that go through the PTC, such as Line 855. By the time 

Foothill negotiated the final contract with Proterra, the per-bus cost had dropped, which enabled 

the agency to purchase 12 buses.

Figure 2. Route map for Line 291 (courtesy of Foothill Transit)
6

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Advanced Technology Bus Development Process—

Technology Readiness Levels

In its 2012 annual status report on fuel cell electric buses,

2 NREL introduced a guideline for 

assessing the technology readiness level (TRL) for FCEBs. This guideline was developed using a 

Technology Readiness Assessment Guide3 published by DOE in September 2011. Although the 

guideline was developed for FCEBs, the same basic concept applies to any advanced technology 

buses including BEBs. Figure 3 provides a graphic representation of the development process.

(Appendix A provides the TRL guideline table tailored for advanced technology bus

commercialization.) The guideline considers the bus as a whole and does not account for 

differing TRLs for separate components or sub-systems. Some sub-systems may include off-theshelf components that are considered commercial, while other sub-systems may feature newly 

designed components at an earlier TRL. 

Figure 3. Graphic representation of the commercialization process developed for FCEBs

BEB development is currently in the technology demonstration/commissioning phase that 

includes TRLs 6 through 8. This phase begins the iterative process to validate the design, analyze 

the results, and reconfigure or optimize the design as needed. The manufacturer typically works 

with a transit agency partner to conduct in-service tests on the bus. Updates to the design are 

made based on the performance results, and the buses go back into demonstration and through 

the cycle until the design meets the performance requirements. This can be a time-consuming 

process as manufacturers work through technical difficulties.

NREL considers the Proterra BEB to be at TRL 7 because the design of the bus is a nextgeneration version based on lessons learned with earlier models and the deployment includes the 

12-bus Foothill fleet as well as numerous other fleets around the country. These buses represent a 

full-scale validation in a relevant environment. Proterra reports that its Catalyst bus is a 3rdgeneration design that is considered to be at TRL 8.

 2 Fuel Cell Buses in U.S. Transit Fleets: Current Status 2012, NREL/TP-5600-56406, 

http://www.nrel.gov/docs/fy13osti/56406.pdf. 3 DOE Technology Readiness Assessment Guide, G 143.3-4a, 

https://www.directives.doe.gov/directives/0413.3-EGuide-04a/view. 
7

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

At this point in the development, BEBs are not fully commercial products. The manufacturers’ 

goals for the demonstration are to verify that the BEB performance meets the technical targets 

and identify any issues that need to be resolved. The capital costs for BEBs are currently higher 

than that of conventional technology, although the costs have dropped considerably over the last 

few years as orders for the buses have increased. The increase in orders allows the manufacturers 

to take advantage of economies of scale to reduce the production costs. Operating costs for BEBs 

are currently low because the buses are all under warranty and the original equipment 

manufacturers are handling much of the repair costs. Once these buses pass the warranty period 

and transit staff takes over the repair work, operating costs are expected to increase.

NREL’s goal in evaluating advanced technology buses is to document the performance and track 

progress over time toward meeting the technical targets. NREL collects data on conventional 

buses at each demonstration site for a baseline comparison. This is important primarily because 

fuel economy is highly dependent on duty cycle, but also because maintenance practices can be 

different from site to site. The best comparisons need to include buses operated in similar service 

at the same operating division. The most accurate comparison would be between buses of the 

same manufacturer, model, production year, and mileage. In that case, the only difference 

between the advanced technology and baseline buses would be the propulsion system. This type 

of baseline comparison is not always possible. 

The Proterra BEBs at Foothill are 35-foot, composite body buses. This is a unique model for the 

transit industry, which makes it a challenge to find a comparable baseline bus. NREL has 

collected data on two groups of baseline buses at Foothill. The agency has a new fleet of 42-foot 

NABI CNG buses, and NREL is collecting data on a selection of these buses for comparison of 

reliability, availability, fuel economy, and maintenance cost. Foothill has also provided fueling 

data on a selection of older, 40-foot CNG buses for fuel economy comparisons.
8

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Bus Technology Descriptions

The BEBs in service at Foothill (Figure 4) are 35-foot, composite body buses built by Proterra. 

As mentioned earlier, selecting a comparable baseline bus for a project can be challenging,

especially when evaluating a unique design such as the Proterra BEB. Foothill does not operate 

conventional buses that are similar in size, weight, and year to the BEBs. The primary baseline 

buses selected are new NABI CNG buses of the same model year as the BEBs. Like the BEBs, 

the NABI CNG buses are under warranty and should have very low maintenance costs. The 

NABI CNG buses have Cummins engines with a three-way catalyst. Table 1 provides bus 

system descriptions for the BEB and CNG buses that were studied in this evaluation. Figure 5

shows one of the NABI CNG baseline buses. For efficiency and fuel economy comparisons, 

Foothill has also provided fueling data from a subset of 40-foot Orion CNG buses. 

Table 1. Fuel Cell and CNG Bus System Descriptions

Vehicle System BEB CNG

Number of buses 12 8

Bus manufacturer/model Proterra/BE35 NABI/BRT-07.03

Model year 2014 2014

Length/width/height 35 ft/102 in./129 in. 42 ft/102 in./137 in.

GVWR/curb weight 37,320 lb/27,680 lb 42,540 lb/33,880 lb

Wheelbase 237 in. 308 in.

Passenger capacity 35 seats, 2 wheelchair 

positions, 18 standees

38 seats, 2 wheelchair 

positions, 10 standees

Motor or engine Permanent magnet, 

UQM, PP220 

CNG engine, Cummins, 

8.9 ISL G

Rated power 220 kW peak (295 hp) 280 hp @ 2,200 rpm

Energy storage

Lithium-titanate 

batteries, 

Altairnano, TerraVolt 

368 V, 88 kWh total 

energy

N/A

Accessories Electric Mechanical

Emissions equipment None 3-way catalyst

Transmission/retarder Regenerative braking N/A

Fuel capacity N/A 7 Type IV cylinders, 

22,204 scf at 3,600 psi 

Bus purchase cost $904,490 $575,000

The warranty for the BEBs is included in the bus purchase cost and covers the following:

• Bumper to bumper – 2 years

• Powertrain subsystem – 5 years

• Major subsystems – 3 years

• Main structure – 12 years

• Battery warranty – 12 years.
9

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Figure 4. Foothill Transit Proterra BEB

Figure 5. Foothill Transit NABI CNG bus

Foothill purchased the buses in 2013 with delivery in 2014. The purchase cost for BEBs 

continues to decrease over time. Foothill reports that the per bus purchase costs for its fleets of 

BEBs has dropped from $1 million to just under $800,000. Table 2 provides costs for the four 

orders of BEBs for Foothill. 

Table 2. Foothill Transit’s BEB Fleet Purchase Cost 

Order 

Number

Number 

of Buses Bus Description Purchase Year

Cost per 

Bus

1 3 35-ft BE35 fast charge buses 2009 $1,000,000

2 12 35-ft BE35 fast charge buses 2013 $904,490

3 2 40-ft Catalyst fast charge buses 2014 $825,000

4 13 40-ft Catalyst extended range buses 2015 $789,000
10

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

The most recent order was eligible for the California Hybrid and Zero-Emission Truck and Bus 

Voucher Incentive Project (HVIP).4 The HVIP was designed to speed up the market introduction 

of clean technologies in the state by providing funds to help offset the incremental cost. 

Incentives through the program bring Foothill’s cost for the most recent order down to $689,000 

per bus.

 4 For more information see the HVIP website at https://www.californiahvip.org/. 
11

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Charging and Maintenance Facilities

The BEBs and CNG buses are operated out of Foothill’s Pomona Operations and Maintenance 

Facility. Foothill provides operation and maintenance of its fleet through contracts with private 

firms. Maintenance staff at the division handles all maintenance work on the CNG buses and 

covers safety inspections, general bus maintenance, and some preventive maintenance for the 

BEBs. Proterra has two on-site technicians that handle all warranty work on the BEBs. At the 

end of each day, operators typically charge the BEBs at the PTC prior to returning to the depot. 

A slow charger is used at the operations and maintenance facility for times when a bus needs 

additional charging. The installation of this slow charger was the only modification needed to 

allow maintenance of the BEBs inside the facility. A new shop charger costs $50,000. Foothill 

plans to eventually add a fast charger at this facility.

Fast-Charge Station 

At the onset of the first BEB project, Foothill worked with the City of Pomona to establish an

on-route fast charging station at the PTC. The site was ideal for the charging station because it 

was at the mid-way point in the route, there was sufficient space to install the equipment, and 

there was a transformer close by. The agency has an agreement with the City of Pomona to lease 

the space at no cost for 40 years. Because the PTC is a transfer point for eight local routes, 

Foothill could potentially expand service of the BEBs to other routes without having to add more 

charging stations.

Availability of the charging station is essential for operation of the buses. When the original 

manufacturer of the chargers dropped the product line from its offerings, finding replacement 

parts became difficult for the team. The funding awarded through the second round of the 

TIGGER Program allowed Foothill to replace the original equipment with two Eaton 500 kW 

chargers and make several other needed upgrades. This included separating the controls for the 

chargers, upgrading cables, and improving wireless communications. Foothill has an agreement 

with Proterra to maintain the chargers and associated equipment. Figure 6 shows a BEB parked 

at the charging station. The building that houses the chargers and equipment is on the left. 

Figure 6. BEB charging station at the PTC
12

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

The Foothill charging station is different from other deployments of Proterra BEBs because it 

has two chargers at one station. Both chargers are housed in the same climate-controlled building 

with charge heads positioned on either side. The two chargers operate as separate units with a 

dedicated control system for each. A common communication network serves both units with 

sensors to detect which charge head a bus is approaching to enable proper bus-to-charger 

communication for docking. Emergency shut-off switches for each charge head are located on 

either side of the building. Figure 7 shows one of the charging heads and an emergency shut-off

switch. 

Figure 7. A charging head (left) and an emergency shut-off control (right) at the PTC

The system can charge two buses simultaneously. Docking a bus with the charging head occurs 

semi-autonomously. The operator does not have to exit the bus to make any connections. The

docking process works as follows:

• The charger’s wireless communication is automatically established as a bus approaches.

• Sensors on the bus and charger detect the presence and location of a bus.

• At approximately 10 feet from the charger, the bus speed is automatically limited. The 

driver only has to steer the bus just as they would approaching any normal bus stop.

• As the bus passes under the charger it automatically comes to a stop and the charge head 

lowers to the roof of the bus.
13

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• The bus moves forward automatically to seat the charge head with the charging 

connection on the roof of the bus.

• The driver sets the parking brake to start the charging process. At this time, there are 

numerous automatic safety checks performed by the system before any current begins to 

flow.

• Passengers can get on and off the bus as they normally would.

• After the batteries are fully charged, the connecting arms automatically retract. 

The system is designed to fully charge a bus in under 10 minutes. For Foothill’s Line 291, 

typical charge times are around 5 minutes. Foothill built a layover time into the schedule to allow 

enough time for charging. Figure 8 shows an up-close photo of one of the Foothill BEBs docked 

with the charging head.

Figure 8. A Foothill BEB docked with the fast charger at the PTC

Costs for the chargers and installation continue to drop. Installation costs will vary from site to 

site depending on a number of factors including the distance to a transformer. The total cost for 

the new charging station being installed at the Azuza park and ride was $998,000. The 

installation includes two 500-kW fast chargers at $349,000 each. The cost to install the chargers 

was $300,000. 
14

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

In-Service Operations Evaluation Results

Foothill Transit put the first of the 12 buses in service in March of 2014. The remaining buses 

were phased into service over the following 4 months. The results presented in this section are 

focused on data from April 2014 through July 2015. During that data period, the BEBs operated 

401,244 miles over 47,462 hours of operation. This indicates an overall operational speed of 8.4

mph. The NABI CNG buses were delivered in late summer of 2014 and went into service around 

late September/early October 2014. The data period for the CNG buses begins in October 2014. 

Route Assignments

Foothill Transit’s BEB fleet operates out of its Pomona Division. The service consists of 21 

routes: 12 local and 9 commuter/express routes. The BEB fleet is operated primarily on Line 

291. Based on Foothill’s schedule for the division, in-service speed for that route is 10.6 mph. 

An analysis of the route from detailed GPS data is included in the next section of the report. The

BEBs are also operated on Line 855 on most mornings. That route runs through the PTC where 

the charger is located. The BEBs are not operated on Line 855 during the afternoons because 

those route blocks include service on commuter routes, which is beyond the current range of the 

buses. The CNG baseline buses are randomly dispatched on all of the routes out of the division 

including commuter routes. Average in-service speed for the Pomona Division as a whole is 17.6 

mph.

Bus Use and Availability

Bus use and availability are indicators of reliability. Lower bus usage may indicate downtime for 

maintenance or purposeful reduction of planned work for the buses. This section summarizes bus 

usage and availability for the BEBs and baseline buses.

Table 3 summarizes the average monthly mileage for the BEB and CNG baseline buses for the 

data period. The average monthly operating mileage for the BEBs for the evaluation period is

2,333 miles, which is about half that of the CNG buses. This is expected, considering that the 

BEBs are operated primarily on Line 291 and the CNG buses are randomly dispatched on all 

routes out of the Pomona Depot, including express and commuter routes, which have much 

higher average speeds. The results presented in this report are based on the planned route for the 

buses and do not indicate a specific limitation of the technology. Figure 9 tracks the monthly 

average miles for the BEBs and CNG buses for the data period. The CNG buses were phased 

into service starting in October 2014.
15

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Table 3. Average Monthly Mileage (Evaluation Period)

Bus Total 

Mileage Months

Average

Monthly

Mileage

BEB Fleet

2004 37,502 16 2,344

2005 37,866 16 2,367

2006 40,532 16 2,533

2007 32,241 13 2,480

2008 32,743 14 2,339

2009 37,052 14 2,647

2010 39,171 14 2,798

2011 28,920 14 2,066

2012 34,998 14 2,500

2013 17,243 14 1,232

2014 31,944 13 2,457

2015 31,032 14 2,217

BEB Total 401,244 172 2,333

CNG Fleet

2200 38,971 10 3,897

2201 46,891 10 4,689

2202 43,657 10 4,366

2203 47,439 10 4,744

2204 46,773 10 4,677

2205 46,605 10 4,661

2206 46,763 10 4,676

2207 47,274 10 4,727

CNG Total 364,373 80 4,555

Figure 9. Monthly average miles for the Foothill BEBs and CNG buses

0

1,000

2,000

3,000

4,000

5,000

6,000

Monthly Average Miles

BEB CNG
16

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Another measure of reliability is availability—the percentage of days the buses are actually 

available out of days that the buses are planned for operation. The data presented are based on 

availability for morning pull-out and don’t necessarily reflect all-day availability. Transit 

agencies typically have a target of 85% availability for their fleets to allow for time to handle 

scheduled and unscheduled maintenance. For the Foothill fleet, the buses are planned to operate 

every day, including weekends. To calculate availability, NREL collected data from several 

sources. One source was the daily service reports that Proterra provides to Foothill. These reports 

list the availability of each BEB for that day and outline any reasons for unavailability. Foothill 

provided approximately 88% of the daily service reports, which is considered to be a sufficient 

sample size to be representative of the total. Foothill also provided the daily garage activity

sheets for the Pomona Depot, which list each bus that is not available for morning pull-out and 

provide a general reason for unavailability. These activity sheets are for the depot as a whole and 

include the BEBs as well as the CNG buses. This was the only source of availability data for the 

CNG buses. The garage activity sheets were not available for every day. During the early data 

period, only 37% of the activity sheets were provided; for the later months of the period (4/15–

7/15), 84% of the activity sheets were provided. Because these are new conventional technology 

buses, the availability is expected to be high. 

Table 4 summarizes the availability for the BEBs during the data period. The per-bus availability 

ranges from a high of 98% to a low of 62%. The overall average for the group is 90%. One 

specific BEB (2013) experienced several issues that kept it out of service for extended periods of 

time. The majority of issues were for general bus problems—repair of accident damage and the 

air conditioning system—and not due to any advanced technology component. Difficulties in 

getting parts for the air conditioning system resulted in longer downtimes than necessary had the 

parts been readily available. None of the other buses had an availability of less than 85%, which 

indicates this bus could be considered an outlier. If this bus is removed from the data set, the 

overall fleet availability is 93%.

Table 4. Summary of Availability by Bus for the BEBs

Bus Planned 

Days

Available 

Days

Percent 

Availability

2004 427 416 97

2005 429 380 89

2006 429 407 95

2007 354 322 91

2008 377 337 89

2009 377 369 98

2010 377 347 92

2011 370 348 94

2012 370 349 94

2013 370 230 62

2014 354 305 86

2015 366 334 91

Total BEB 4,600 4,144 90

adjusted (w/o 2013) 4,230 3,914 93
17

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Figure 10 tracks the monthly availability for the BEBs (dark blue line) and CNG buses (green

line) for the data period. The adjusted average availability for the BEBs without 2013 is also 

shown in the graph (light blue line) to indicate how the extended downtime for that one bus 

affected the data. The figure also provides an indication of the reasons for unavailability. The 

stacked bars for each month show the number of days the BEBs were not available by five

categories. The majority of issues causing downtime for the buses were related to bus systems. 

The availability of the CNG buses dropped significantly during the last 2 months of the data 

period. One of the baseline buses was involved in an accident and has been out of service for 

major body work.

Figure 10. Availability for the BEBs and CNG buses

Table 5 summarizes the reasons for unavailability for the BEBs and CNG buses. During this 

reporting period, the average availability was 90% for the BEBs and 94% for the CNG buses. 

Bus-related maintenance is the reason for the highest percentage of unavailability for the BEBs 

and CNG buses. Figure 11 presents the overall reasons for unavailability for the BEBs in a pie 

chart.

0

10

20

30

40

50

60

70

80

90

100

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Apr-14

May-14

Jun-14

Jul-14

Aug-14Sep-14

Oct-14

Nov-14

Dec-14

Jan-15

Feb-15

Mar-15

Apr-15

May-15

Jun-15

Jul-15

Number of days

Availability

ESS

Charging

issues

Electric drive

PM

Bus

BEB

CNG

Target

w/o 2013

Reasons for 

Unavailability
18

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Table 5. Summary of Availability and Unavailability of Buses for Service

Category BEB

# Days

BEB

%

CNG

# Days

CNG

%

Planned work days 4,600 1,360

Days available 4,144 90 1,283 94

Unavailable 456 77

ESS 40 9 — —

CNG engine — — 5 6

Electric drive 101 22 — —

Charging issues 6 1 — —

Preventive maintenance 8 2 12 16

General bus maintenance 301 66 54 70

Transmission 0 0 6 8

Figure 11. Reasons for unavailability for the Foothill BEBs

Energy Use, Fuel Economy, and Cost

The BEBs are typically charged each time the bus stops at the PTC. Figure 12 shows the total 

energy used and number of charges for the BEB fleet by month. The figure shows the ramp-up in 

use of the fleet as the buses were delivered and placed into service. Once all of the buses were in 

service (July 2014), the fleet averaged 61,174 kWh and 3,131 charges per month.

Proterra records and stores data—including total kWh, number of charges, and miles driven—on 

each of the buses. These data were provided to NREL for calculating efficiency of the buses in 

kWh per mile. Foothill’s CNG buses are typically fueled once each day. Foothill provided 

individual fueling records for the CNG buses. CNG is typically tracked in units of gasoline 

gallon equivalent (GGE). NREL used these records to calculate the CNG fuel economy in miles 

per GGE as well as miles per diesel gallon equivalent (DGE). To compare the BEBs to the 

baseline buses, NREL converted the kWh to DGE using a conversion factor of 37.7 kWh/gallon.

The explanation of the energy conversion from kWh of electricity to DGE appears at the end of 

Appendix B. (Appendices B and C contain summary statistics for the BEB and CNG buses.)

Bus

66%

PM

2%

Electric 

drive

22%

Charging 

issues

1%

ESS

9%
19

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Figure 12. Monthly energy use and number of charges for the BEB fleet

Table 6 shows electricity and CNG fuel consumption and fuel economy for the study buses 

during the reporting period. The BEBs had an overall average efficiency of 2.15 kWh per mile, 

which equates to 17.48 miles per DGE. The CNG buses have an average fuel economy that is 

4.04 miles per GGE, which equates to 4.51 miles per DGE. These results indicate that the BEBs

have an average fuel economy that is nearly 4 times higher than that of the CNG buses.

0

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

0

10,000

20,000

30,000

40,000

50,000

60,000

70,000

80,000

Energy used (kWh)

Number of Charges

Energy use kWh Number of charges
20

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Table 6. Energy Use and Fuel Economy (Evaluation Period)

Bus Mileage

(fuel base)

Energy 

(kWh) or 

CNG (GGE)

kWh per 

Mile or 

Miles per 

GGE

Diesel Equivalent 

Amount (Gallon)

Miles per 

Gallon 

(DGE)

BEB Fleet

2004 37,502 80,818 2.16 2,146 17.47

2005 37,866 82,451 2.18 2,190 17.29

2006 40,532 86,822 2.14 2,306 17.58

2007 32,241 70,386 2.18 1,869 17.25

2008 32,743 71,473 2.18 1,898 17.25

2009 37,052 79,184 2.14 2,103 17.62

2010 39,171 81,101 2.07 2,154 18.19

2011 28,920 60,815 2.10 1,615 17.91

2012 34,998 75,647 2.16 2,009 17.42

2013 17,243 36,398 2.11 967 17.84

2014 31,944 67,492 2.11 1,792 17.82

2015 31,032 71,999 2.32 1,912 16.23

BEB Total 401,244 864,586 2.15 22,959 17.48

CNG Fleet

2200 31,407 7,785 4.03 6,967 4.51

2201 39,567 9,816 4.03 8,785 4.50

2202 34,540 8,189 4.22 7,329 4.71

2203 36,537 9,631 3.79 8,620 4.24

2204 36,591 9,200 3.98 8,234 4.44

2205 37,894 9,360 4.05 8,377 4.52

2206 36,342 8,915 4.08 7,979 4.55

2207 41,655 10,083 4.13 9,024 4.62

CNG Total 294,533 72,979 4.04 65,315 4.51

Figure 13 shows monthly average fuel economy for the BEBs and CNG buses in miles per DGE. 

The monthly average high temperature is included in the graph to track any seasonal variations 

in the fuel economy due to heating or cooling of the bus, which might require additional energy 

use. The fuel economy for the 40-foot Orion bus fleet is also included for comparison. The 

average fuel economy for the Orion fleet is 4.4 miles per DGE, which is not significantly 

different from the NABI 45-foot CNG buses.
21

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Figure 13. Monthly average fuel economy for the BEBs and CNG buses (evaluation period)

Foothill’s charging station was installed with a separate utility meter. Foothill provided the 

monthly utility bills to NREL to determine the energy cost. During the evaluation period, the 

electricity cost averaged $0.18 per kWh. The electricity cost for the fleet calculates to $0.39 per 

mile. The CNG fuel cost during the reporting period averaged $0.93 per GGE, which calculates 

to $0.23 per mile for the NABI CNG buses.

10

20

30

40

50

60

70

80

90

100

0

2

4

6

8

10

12

14

16

18

20

Average High Temperature (F)

Fuel Economy (mpDGE)

BEB (35-ft) CNG (Orion, 40-ft) CNG (NABI, 42-ft) Avg High Temp (F)
22

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Roadcall Analysis

A roadcall or revenue vehicle system failure (as named in the National Transit Database5) is 

defined as a failure of an in-service bus that causes the bus to be replaced on route or causes a 

significant delay in schedule.6 If the problem with the bus can be repaired during a layover and 

the schedule is kept, this is not considered a roadcall. The analysis described here includes only 

roadcalls that were caused by “chargeable” failures. Chargeable roadcalls include systems that 

can physically disable the bus from operating on route, such as interlocks (doors, air system), 

engine, or things that are deemed to be safety issues if operation of the bus continues. They do 

not include roadcalls for things such as problems with radios, fareboxes, or destination signs.

The transit industry measures reliability as mean distance between failures, also documented as 

miles between roadcalls (MBRC). Table 7 provides the MBRC for the BEBs and CNG buses 

categorized by bus roadcalls, propulsion-related roadcalls, and ESS-related roadcalls. 

Propulsion-related roadcalls include all roadcalls due to propulsion-related systems including the 

battery system (or engine for a conventional bus), electric drive, fuel, exhaust, air intake, cooling, 

non-lighting electrical, and transmission systems. The ESS-related roadcalls and MBRC are 

included for the BEBs. 

Table 7. Roadcalls and MBRC (Evaluation Period)

BEB CNG

Dates 4/14–7/15 10/14–7/15

Mileage 401,244 364,373

Average miles accumulated per bus 33,437 45,547

Bus roadcalls 43 8

Bus MBRC 9,331 45,547

Propulsion-related roadcalls 16 4

Propulsion-related MBRC 25,078 91,093

ESS-related roadcalls 3

ESS-related MBRC 133,748

Figure 14 presents the cumulative MBRC by category for the BEBs and CNG baseline buses. 

DOE and FTA have not established performance targets specific to electric drive buses. The 

MBRC targets for FCEBs were based on typical conventional buses and could be considered 

appropriate for any advanced technology. The ultimate target for bus MBRC (4,000) is included 

in the figure as a black dotted line. The ultimate target for fuel-cell-system-related MBRC is 

20,000. This is considered comparable to roadcalls for BEBs that are related to the battery, or 

ESS. This is shown in the figure as a dashed red line designated ESS MBRC Target. At this stage 

of demonstration, the Foothill BEBs have achieved an ESS MBRC that is significantly higher 

than the target.

 5 National Transit Database website: www.ntdprogram.gov/ntdprogram/. 6 AC Transit defines a significant delay as 6 or more minutes.
23

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Figure 14. Cumulative MBRC for the BEBs and CNG buses

Maintenance Analysis

All work orders for the study buses were collected and analyzed for this evaluation. For 

consistency, the maintenance labor rate was kept at a constant $50 per hour; this does not reflect 

an average rate for Foothill Transit. Costs for accident-related repair, which are extremely 

variable from bus to bus, were eliminated from the analysis. This section first covers total 

maintenance costs and then maintenance costs by bus system. Warranty costs are not included in 

the cost-per-mile calculations. All of the buses are currently under warranty.

At the beginning of the project, all maintenance on the BEBs was performed by Proterra 

technicians. In January 2015, the Foothill contractor staff took over the preventive maintenance 

inspections (PMI) and general bus work. Because of this, the work orders provided on the BEBs 

cover a different time period than the overall evaluation period. To calculate maintenance cost 

per mile, NREL used the mileage accumulated from January 2015 through July 2015.

Total Work Order Maintenance Costs

Total maintenance costs include the price of parts and labor rates at $50 per hour. Cost per mile 

is calculated as follows:

Cost per mile = [(labor hours * 50) + parts cost] / mileage

Table 8 shows total maintenance costs for the BEBs and CNG buses. Scheduled and unscheduled 

maintenance cost per mile is provided for each bus and study group of buses.

0

50,000

100,000

150,000

200,000

250,000

Jul-14 Aug-14 Sep-14 Oct-14 Nov-14 Dec-14 Jan-15 Feb-15 Mar-15 Apr-15 May-15 Jun-15 Jul-15

Miles Between Roadcall

BEB Bus MBRC

BEB Propulsion MBRC

BEB Battery MBRC

CNG Bus MBRC

CNG Propulsion MBRC

Bus MBRC Target

ESS MBRC Target
24

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

During the reporting period, the BEBs had a maintenance cost per mile that was slightly lower 

(11% less) than that of the CNG buses.

Table 8. Total Work Order Maintenance Costs (Evaluation Period)

Bus Mileage Parts ($) Labor 

Hours

Total Cost 

per Mile 

($)

Scheduled 

Cost per 

Mile ($)

Unscheduled 

Cost per Mile 

($)

BEB Fleet

2004 16,191 $1,124.69 45.0 $0.21 $0.09 $0.12

2005 16,581 $1,262.52 63.5 $0.27 $0.07 $0.20

2006 18,916 $405.35 53.7 $0.16 $0.08 $0.09

2007 19,120 $1,447.90 50.5 $0.21 $0.10 $0.11

2008 17,817 $788.72 32.7 $0.14 $0.06 $0.08

2009 18,254 $495.90 37.5 $0.13 $0.08 $0.05

2010 18,630 $1,621.40 33.8 $0.18 $0.06 $0.12

2011 15,881 $0.00 35.5 $0.11 $0.09 $0.02

2012 18,330 $394.36 37.7 $0.12 $0.08 $0.05

2013 11,409 $11.00 35.5 $0.16 $0.11 $0.05

2014 17,321 $12.36 26.0 $0.08 $0.06 $0.02

2015 16,591 $1,780.52 31.0 $0.20 $0.06 $0.14

Total BEB 205,041 $9,344.72 482.4 $0.16 $0.08 $0.09

CNG Fleet

2200 38,971 $2,747.20 67.5 $0.16 $0.12 $0.04

2201 46,891 $5,354.20 94.9 $0.22 $0.18 $0.04

2202 43,657 $4,426.53 108.3 $0.23 $0.16 $0.07

2203 47,439 $3,771.99 85.5 $0.17 $0.14 $0.03

2204 46,773 $4,826.42 93.5 $0.20 $0.15 $0.06

2205 46,605 $3,932.59 81.8 $0.17 $0.15 $0.02

2206 46,763 $4,442.17 90.3 $0.19 $0.14 $0.05

2207 47,274 $3,260.65 57.8 $0.13 $0.09 $0.04

Total CNG 364,373 $32,761.73 679.5 $0.18 $0.14 $0.04

The monthly scheduled and unscheduled maintenance cost per mile for the BEBs is shown in 

Figure 15. The average monthly odometer is included on the graph. Figure 16 provides the same 

information for the CNG buses. Scheduled maintenance for the BEBs was fairly consistent over 

the data period. The higher unscheduled maintenance during May and June was primarily due to 

tire replacement on four buses and low-voltage battery replacement on three buses. Foothill 

reports that the tire damage is not typical of its fleet. A broken curb at one of the stops on Line 

291 has resulted in tire and fender damage as drivers stop to pick up passengers. Monthly 

scheduled and unscheduled maintenance costs for the CNG buses are generally consistent until 

July 2015, when all eight buses reached the mileage for a major PMI.
25

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Figure 15. Monthly scheduled and unscheduled maintenance costs per mile for the BEBs

Figure 16. Monthly scheduled and unscheduled maintenance costs per mile for the CNG buses

0

10,000

20,000

30,000

40,000

50,000

60,000

$0.00

$0.10

$0.20

$0.30

$0.40

$0.50

$0.60

$0.70

Average odometer

Cost per Mile

Unscheduled Scheduled Average odometer

0

10,000

20,000

30,000

40,000

50,000

60,000

$0.00

$0.10

$0.20

$0.30

$0.40

$0.50

$0.60

$0.70

Average odometer

Cost per Mile

Unscheduled Scheduled Average odometer
26

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Work Order Maintenance Costs Categorized by System 

Table 9 shows maintenance costs by vehicle system and bus study group (without warranty 

costs). The vehicle systems shown in the table are as follows:

• Cab, body, and accessories: Includes body, glass, and paint repairs following accidents; 

cab and sheet metal repairs on seats and doors; and accessory repairs such as 

hubodometers and radios

• Propulsion-related systems: Repairs for exhaust, fuel, engine, electric motors, battery 

modules, propulsion control, non-lighting electrical (charging, cranking, and ignition), air 

intake, cooling, and transmission

• PMI: Labor for inspections during preventive maintenance

• Brakes

• Frame, steering, and suspension

• HVAC

• Lighting

• Air system, general

• Axles, wheels, and drive shaft 

• Tires.

Table 9. Work Order Maintenance Cost per Mile by System (Evaluation Period)

System

BEB CNG

Cost 

per 

Mile 

($)

Percent 

of Total 

(%)

Cost 

per 

Mile 

($)

Percent 

of Total 

(%)

Propulsion-related 0.02 13 0.08 42

Cab, body, and accessories 0.03 17 0.02 10

PMI 0.07 44 0.06 33

Brakes 0.00 0 0.00 0

Frame, steering, and suspension 0.00 0 0.00 0

HVAC 0.00 0 0.00 2

Lighting 0.00 0 0.00 0

Air, general 0.00 0 0.02 9

Axles, wheels, and drive shaft 0.00 2 0.00 1

Tires 0.04 24 0.01 3

Total 0.16 100 0.18 100

The systems with the highest percentage of maintenance costs for the BEBs were PMI; cab, 

body, and accessories; and propulsion-related. For the NABI CNG buses the systems with the 

highest percentage of maintenance costs were propulsion-related; PMI; and cab, body, and 

accessories. Figure 17 shows the monthly maintenance cost per mile by category for the BEBs.

Figure 18 presents the same data for the CNG buses.
27

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Figure 17. Monthly maintenance cost per mile by category for the BEBs

Figure 18. Monthly maintenance cost per mile by category for the CNG buses

Propulsion-Related Work Order Maintenance Costs

Propulsion-related vehicle systems include the exhaust, fuel, engine, battery modules, electric 

propulsion, air intake, cooling, non-lighting electrical, and transmission systems. These systems 

have been separated to highlight maintenance costs most directly affected by the advanced 

propulsion system changes for the buses. Table 10 shows the propulsion-related system repairs 

by category for the two study groups during the reporting period. During the data period, the 

propulsion-related maintenance costs for the BEBs were 73% lower than that of the CNG buses; 

however, the costs for both groups were low overall.

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

Cost per Mile

Tires

General air system repairs

Axles, wheels, and drive shaft

Lighting

HVAC

Frame, steering, and

suspension

Brakes

PMI

Propulsion-related

Cab, body, and accessories

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

Cost per Mile

Tires

General air system repairs

Axles, wheels, and drive shaft

Lighting

HVAC

Frame, steering, and

suspension

Brakes

PMI

Propulsion-related

Cab, body, and accessories
28

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Table 10. Propulsion-Related Work Order Maintenance Costs by System (Evaluation Period)

Maintenance System Maintenance Costs BEB CNG

Total mileage 205,041 364,373

Average miles per bus 17,087 45,546

Total propulsionrelated systems

(roll-up)

Parts cost ($) 2,421.01 23,240.22

Labor hours 37.0 94.8

Total cost ($) 4,271.01 27,977.72

Total cost ($) per mile 0.02 0.08

Exhaust system repairs

Parts cost ($) 0.00 0.00

Labor hours 0.0 0.0

Total cost ($) 0.00 0.00

Total cost ($) per mile 0.00 0.00

Fuel system repairs

Parts cost ($) 0.00 64.68

Labor hours 0.0 6.5

Total cost ($) 0.00 389.68

Total cost ($) per mile 0.00 0.00

Power plant system 

repairs (battery system 

or CNG engine)

Parts cost ($) 0.00 15,590.27

Labor hours 10.5 25.5

Total cost ($) 525.00 16,865.27

Total cost ($) per mile 0.00 0.05

Electric motor and 

propulsion system 

repairs

Parts cost ($) 0.00 0.00

Labor hours 1.5 0.0

Total cost ($) 75.00 0.00

Total cost ($) per mile 0.00 0.00

Non-lighting electrical 

system repairs (general 

electrical, charging, 

cranking, ignition)

Parts cost ($) 2,421.01 1,158.84

Labor hours 24.5 29.0

Total cost ($) 3,646.01 2,608.84

Total cost ($) per mile 0.02 0.01

Air intake system 

repairs

Parts cost ($) 0.00 4,427.07

Labor hours 0.0 0.8

Total cost ($) 0.00 4,464.57

Total cost ($) per mile 0.00 0.01

Cooling system repairs

Parts cost ($) 0.00 1,810.22

Labor hours 0.0 24.5

Total cost ($) 0.00 3,035.22

Total cost ($) per mile 0.00 0.01

Transmission system 

repairs

Parts cost ($) 0.00 189.15

Labor hours 0.5 5.0

Total cost ($) 25.00 439.15

Total cost ($) per mile 0.00 0.00

Hydraulic system 

repairs

Parts cost ($) 0.00 0.00

Labor hours 0.0 3.5

Total cost ($) 0.00 175.00

Total cost ($) per mile 0.00 0.00
29

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Detailed Technology Evaluation Results

NREL’s Fleet Test and Evaluation team has found medium-duty and heavy-duty vehicle fleets to 

be good candidates for deploying low-emitting advanced technologies due to their large 

numbers, high vehicle miles traveled—and consequent high petroleum fuel consumption and 

emissions—and frequent operation in large population centers, as well as common return-to-base 

fueling regimes and consistent driving routes. 

Previous testing and analysis conducted by NREL have illustrated the influence of drive cycle 

and vehicle usage on both energy consumption (from liquid fuels and high-voltage hybrid battery 

packs) and exhaust (or well-to-wheels) emissions. Drive cycle has also been shown to influence 

the all-electric range of battery electric vehicles, the charge-depleting range of plug-in hybrid 

electric vehicles, and the potential fuel economy benefit of hybrid electric vehicles. Accordingly, 

fleet customers can benefit from a further understanding of advanced vehicle technology 

deployment to minimize fuel consumption and emissions. Large-scale deployments of electric 

vehicles in a localized area can lead to power quality and power cost issues due to increased peak 

demand.

Under DOE funding, NREL’s Fleet Test and Evaluation team partnered with Proterra to perform 

a detailed performance evaluation of its BEBs in use at Foothill Transit. Proterra provided the inuse raw vehicle data for this analysis as Proterra collects and maintains detailed (2 Hz) data for 

monitoring and development work. 

These data will be used to understand the overall usage and effectiveness of the BEBs in transit 

fleet operations and will be compared to operations of conventional counterparts in the same 

location. Through this collaboration with Proterra, NREL hopes to provide a more focused 

investigation to understand the implementation and performance of BEBs.

Detailed Data Collection Approach

Data were received electronically from Proterra via a secure File Transfer Protocol site starting 

in the summer of 2015. A total of 774 days of operational data were provided for 12 buses during 

the following four periods throughout the year (to account for any seasonal variation).

• July–August 2014

• October–November 2014

• January–February 2015

• April–May 2015.

Data channels include details on vehicle speed; GPS location; battery pack SOC; battery pack 

current; battery pack voltage; inverter current; inverter voltage; motor HVAC status; and more. 

These data are being used to assess vehicle and component performance; conduct detailed drive 

cycle analysis; and build and refine vehicle models for further analysis.

Battery lifetime uncertainty is a major barrier to fleet manager decisions regarding the adoption 

of electric vehicles of all types. To reduce lifetime uncertainty, NREL and Proterra are 
30

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developing a study to perform benchmarking tests of Proterra’s BE35 battery packs at regular 

stages throughout their life.

Proterra is currently finalizing its in-field battery pack health test procedure, which will be used 

on every vehicle at least every 12 months and at every individual customer facility at least once 

per quarter. NREL will perform an independent test to validate this procedure, and then Proterra 

will provide the results of the in-field testing from the Foothill Transit buses.

The battery health benchmarking test results will provide valuable in-use data. These data will 

allow NREL to better quantify battery pack health and track battery performance changes over a 

lifetime as well as validate battery life assumptions to help develop a fleet business case.

Results 

Initial analysis was performed on the location-based data to ensure data integrity and to better 

understand the operation of the BEBs on an electrified route. Figure 19 shows the GPS data for

Foothill Transit Line 291 with the fast charger located approximately in the middle at the PTC. 

This route consists of two loops, a 9.3-mile northern loop and a 6.8-mile southern loop, for a 

total distance of 16.1 miles.

Figure 19. Foothill Transit Line 291 route shown with GPS data

The location of the fast charger stations at the PTC can be seen in Figure 20 as green traces of 

bus activity pulling into the charging station.
31

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Figure 20. Fast charger location on Foothill Transit’s Line 291

NREL used its Drive-Cycle Rapid Investigation, Visualization, and Evaluation Analysis 

(DRIVE) tool to calculate preliminary drive cycle statistics for all 774 driving days. These drive 

cycle statistics are provided in Table 11.

Table 11. Foothill Transit Daily Drive Cycle Statistics

Daily Average Foothill Transit

Average speed (including idle time) (mph) 8.42

Average driving speed (mph) 17.66

Kinetic intensity (1/mi) 1.71

Stops per mile 3.70

Number of stops per day 444

Distance traveled (miles) 119.30

Daily hours of operation (hours) 13.9 total / 6.72 driving

Acceleration (ft/s2) 1.44

Deceleration (ft/s2) −1.81

Percent of total time charging 5.56%

Additional drive cycle metrics were also calculated for the Foothill Transit data set as well as for 

the existing NREL Fleet DNA mass transit data set. Figure 21 shows the kinetic intensity vs.

average driving speed for these two data sets as well as for four standard chassis dynamometer 

test cycles. Kinetic intensity is a measure of drive cycle kinetics to define how much ‘stop and 
32

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go’ is in a drive cycle; it is derived from the ratio of aerodynamic speed and characteristic 

acceleration.7 Drive cycles with higher kinetic intensity, with lots of ‘stop and go’ events, offer 

more opportunities for regenerative braking energy recapture through hybridization or 

electrification, while drive cycles with higher steady state speeds, and lower kinetic intensity are 

more likely to benefit from aerodynamic improvements.

Figure 21. Kinetic intensity shown as a function of average driving speed for all Foothill Transit 

data (purple) and all Fleet DNA mass transit data (blue) compared to select standard chassis 

dynamometer test cycles

When comparing the Foothill Transit data set to the existing Fleet DNA mass transit database, 

one can see that on average the Foothill Transit buses stop more frequently and, as shown in 

Figure 22, have a more consistent (tightly clustered) pattern on a per-mile basis than the other 

vehicles do.

 7 M. O’Keefe, A. Simpson, K. Kelly, and D. Pedersen, “Duty cycle characterization and evaluation towards heavy 

hybrid vehicle applications,” SAE Technical Paper 2007-01-0302, 2007. doi:10.4271/2007-01-0302.

http://www.nrel.gov/docs/gen/fy07/40929.pdf.
33

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Figure 22. Kinetic intensity as a function of daily average stops per mile for both the Fleet DNA 

mass transit database and the Foothill Transit BEBs

Further investigation into the operation of these BEBs confirms that these buses not only stop 

quite often when on route, they also spend very little time at the fast charging stations while their 

battery packs are recharged. Figure 23 shows a single 16.1-mile loop on Line 291 with three 

separate charging events. The green line shows the cumulative distance for the day. Between 

charges, the battery pack SOC fluctuates between approximately 30% and 80%.

Figure 23. Vehicle speed, distance, and battery pack SOC shown for one full “loop” on Line 291
34

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Figure 24 shows a histogram of the average battery pack SOC. For the majority of the time the 

battery pack is between 60% and 90% SOC with a mean of 75.4% and a standard deviation of 

11.3%. This indicates there may be an opportunity to shift the usage window to a lower SOC on 

average to help prolong battery pack life.

Figure 24. Average battery pack SOC for all vehicles sampled at 2 Hz

More detailed analysis will be performed on power electronics and accessory loads in the future.

Figure 25 shows an example of a bus heater cycling on and off while stationary, allowing for 

isolation of this 8.3-kW load.

Figure 25. Auxiliary load analysis example showing the power impact of the resistive element 

heater during stationary use
35

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Figure 26 shows a “heat map” illustrating the relationship between calculated battery power and 

vehicle speed. In this figure, positive power represents charging or regenerative braking, and 

negative power is power being drawn from the pack. There are two areas of interest in this 

figure. The first is at zero speed where there is a concentration of power going into the pack. The 

second is at 20 mph, which shows a higher concentration of regeneration braking energy into the 

pack during at what appears to be the downshifting sequence where the transmission holds speed 

for a brief amount of time while the shift occurs. 

Figure 26. Heat map showing the distribution of battery power as a function of vehicle speed 

(positive power indicates energy into the pack)

Detailed Analysis Summary Findings

Additional analysis is still required to draw significant conclusions about the technology, but the 

findings to date include the following:

• The average energy efficiency is 2.15 kWh per mile over 399,663 miles of use.

• The average battery pack SOC is equal to 75.4%, indicating a possibility for a usage 

window shift.

• The average runtime per day is 13.2 hours with an average of 13 charges per day. Each 

charge averages 20 kWh energy delivered.

• Accessory loads contribute to the overall range capability, as more than 50% of “system 

on” time is spent at a speed of 0 mph where lighting and HVAC loads are still required. 
36

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Summary of Achievements and Challenges

This section focuses on the achievements and challenges of Foothill Transit and its partners in 

implementing BEBs into the fleet. As with all new technology development, there have been 

many lessons learned that can be used by other agencies considering BEB technology. There 

have been many achievements for the demonstration, including the following:

• The project deployed 12 fast-charge buses to fully electrify one route. The selected route, 

Line 291, is considered an optimal route for the technology.

• The BEB fleet operated well, accumulating more than 401,000 miles (through July 2015).

• There were no major issues with the advanced technology components. Bus-related 

components were the cause of the majority of issues.

• To date, the BEBs have proved very reliable. Bus MBRC for the data period was more 

than 9,000; propulsion-related MBRC was more than 25,000.

• The on-route fast chargers operated reliably with minimal issues, none of which resulted 

in downtime for the buses.

• The project team took a proactive approach to technical problems, capturing issues in the 

early stages and implementing long-term solutions.

Advanced technology demonstrations typically experience challenges and issues that need to be 

resolved. The challenges and lessons learned from the demonstration included bus-related 

problems as well as programmatic issues. The remainder of this section summarizes the primary 

issues that affected the demonstration as a whole, beginning with the programmatic issues 

followed by the technical issues.

Electricity cost/demand charges—One major challenge for Foothill is addressing demand 

charges that increase electricity cost. Foothill is subject to tiered rates, depending on use, and

demand charges. Anticipating this issue when the first BEB demonstration began, Foothill 

negotiated a waiver for demand charges from its utility provider (Edison). With an increase from 

3 to 15 BEBs, the utility will not renew the waiver when it expires at the end of 2015. Foothill is 

working with Edison on a new agreement to set a reasonable rate charge. This will be a major 

challenge for any fleet looking to deploy electric buses that charge during peak times. The 

industry needs to work on a permanent solution for all BEB adopters to keep costs reasonable in 

the future.

Training drivers for docking the BEBs at the chargers—Operator training was a challenge 

because the procedure for docking with the charger is very different from the process of driving 

conventional buses. During the docking procedure, the operator needs to apply the accelerator

instead of the brake, which is the opposite of the process for pulling up to a stop with a CNG or 

other conventional bus. The project team recommends providing thorough and ongoing driver 

training for new technology buses.

Evolution of technology and components—The project partners have experienced issues 

similar to other advanced technologies where manufacturers change or components are modified 

or discontinued. When the original supplier for the charger dropped the product from its line, the 
37

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team was forced to find another supplier. This increased the overall project costs and had the 

potential to disrupt service of the buses while the installation was being completed. 

Emergency stop button activation—One issue that occurred in the early stages of the project 

was with the emergency stop button at the charging station. Occasionally someone would push 

the button, disabling the charger. This required someone at the site to physically reset the system. 

To discourage this situation, Foothill added a cover over the button and a sign to indicate the area 

was under surveillance. This has been successful so far in deterring troublemakers from 

activating the emergency stop. The team also made modifications so the site can be reset 

remotely to avoid having to send staff to the site.

Tire and body damage—Tire damage from a cracked curb at one of the stops on Line 291 

resulted in unexpected repair costs. Since the buses went into service, Foothill has replaced 19 

tires at a parts cost of more than $6,700 and 25 labor hours. In some cases, the fenders were also 

damaged. The agency has reported the issue to local officials and requested that the curb be 

repaired to prevent future problems.

HVAC issues—The BEBs developed a few issues with the HVAC system, primarily due to 

refrigerant leaks. On one bus, the HVAC compressor failed. This bus experienced extended 

downtime for the repair because of parts supply issues and because the replacement compressors 

also failed. 

Traction motor inverter—A few buses experienced issues with inverter failure due to poor 

cooling distribution. Proterra increased the coolant flow through the inverter to address the issue. 

This modification was rolled out to the entire fleet of buses at Foothill as well as other locations.

Starter battery issues—The low-voltage starter batteries on several of the buses had to be 

replaced during the data period. The primary cause was considered a driver training issue. 

Operators would fail to shut the bus down at the end of a shift, likely because they could not hear 

it running. After several occurrences, the batteries would need to be replaced. 

Battery system updates—As part of the battery management system, a local monitoring unit is 

connected to each module to track battery conditions. Several buses in the fleet experienced 

failure of this system that was traced to the low-voltage connection between the local monitoring 

unit and the battery module. Proterra replaced the wires for all the local monitoring unit/module 

connections.

Parts availability issues—As with other advanced technology buses, the project partners have 

experienced issues with availability of parts. The majority of issues were with components from 

outside suppliers, such as the HVAC compressor. 

Lessons Learned

Foothill and Proterra report that the project went extremely well because they worked carefully 

up front to plan and execute the project. This included engaging local officials early on to work 

through any issues with siting the charging station. Officials with the City of Pomona were 

supportive of the project and cooperated on the charger installation at the PTC. The team has 
38

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identified several key lessons learned during the early implementation that could be helpful for 

other transit agencies considering electric bus technology.

• Plan ahead to determine what organizations need to be engaged in the early stage of 

implementation. Key stakeholders can include utilities, local city officials, first 

responders, and the general public. 

• Review potential routes and consider the ones that best fit how BEBs operate.

• Adjust route schedules to accommodate BEB charging time; this is part of the transition 

from conventional technology buses to electric buses. The starting point for a route also 

may need to be adjusted depending on the location of the charging station(s).

Foothill reports that there is still a lot of learning as the agency ramps up to a larger BEB fleet. 

The team needs to understand how service can transition to a higher number of buses. Charger 

availability is imperative for successful deployment. 
39

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

What’s Next

Foothill Transit will continue operating the 12 BEBs on Line 291 and is evaluating other routes 

that might be well suited for electric buses. The agency has orders for Proterra’s new Catalyst 

40-foot BEB. The first two will be fast-charge buses, followed by 13 of the Catalyst XR 

extended range model BEBs. 

Foothill is looking at potential routes for the 40-foot buses. The agency plans to operate the 

buses on Line 286, a 22-mile round trip. Foothill is also building a new transit center with a park 

and ride in partnership with the LA Metro Gold Line. The transit center will include two

chargers, which will expand the number of routes that the BEBs could operate on.

Future Analysis

NREL plans to continue the in-service performance evaluation for another year and expects to 

publish a second report in late 2016. The data available from Foothill and Proterra align well

with the NREL data collection protocol. Using the data provided, NREL is able to analyze all of 

the key performance metrics in comparison to conventional technology. NREL does not typically 

collect data on warranty repairs for the buses. These repairs are covered by the manufacturer and 

are part of the purchase price of the bus. It could be beneficial to collect these data to give an 

indication of the future maintenance costs of the buses.

In continuation of the DOE-funded Fleet Test and Evaluation project, NREL will deploy vehicle 

data loggers on CNG buses operating on a variety of routes to collect in-use operational data on 

baseline buses. These data will allow researchers to characterize the different routes and drive 

cycles as well as compare the operational performance of the CNG buses. Additionally NREL 

has created a power-based vehicle model for the Proterra BE35 that will be used to evaluate the 

feasibility of operating electric buses on additional routes. 

NREL is also very interested in performing chassis dynamometer testing of the BEBs and the 

CNG buses and hopes to precisely quantify both energy and emissions savings in a controlled 

environment on a variety of drive cycles.
40

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Contacts

California Air Resources Board

1001 I Street

P.O. Box 2815

Sacramento, CA 95812

Yachun Chow, Manager, Zero Emission Truck and Bus Section

Phone: 919-322-7450

Email: Yachun.Chow@arb.ca.gov

NREL

15013 Denver West Parkway

Golden, CO 80401

Leslie Eudy, Senior Project Leader, Technology Validation Team

Phone: 303-275-4412

Email: leslie.eudy@nrel.gov

Ken Kelly, Section Supervisor/Project Leader, Fleet Test and Evaluation Team

Phone: 303-275-4465

Email: kenneth.kelly@nrel.gov

Foothill Transit

100 S. Vincent Ave.

Suite 200

West Covina, CA 91790

Roland Cordero, Director of Maintenance and Vehicle Technology

Phone: 626-931-7246

Email: rcordero@foothilltransit.org

Proterra

1 Whitley Court

Greenville, SC 29607

Mike Finnern, Customer Service Director

Phone: 864-214-0393

Email: MFinnern@Proterra.com
41

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

References and Related Reports

All NREL hydrogen and fuel cell-related evaluation reports can be downloaded from the 

following website: www.nrel.gov/hydrogen/proj_fc_bus_eval.html.

O’Keefe, M.; Simpson, A.; Kelly, K.; Pedersen, D. “Duty cycle characterization and evaluation 

towards heavy hybrid vehicle applications.” SAE Technical Paper 2007-01-0302, 2007. 

doi:10.4271/2007-01-0302. http://www.nrel.gov/docs/gen/fy07/40929.pdf. 

Eudy, L.; Post, M.; Gikakis, C. (2015). Fuel Cell Buses in U.S. Transit Fleets: Current Status 

2015. NREL/TP-5400-64974. Golden, CO: National Renewable Energy Laboratory.

Eudy, L. (2010). Fuel Cell Transit Bus Evaluations, Joint Evaluation Plan for the U.S. 

Department of Energy and the Federal Transit Administration. NREL/TP-560-49342. Golden, 

CO: National Renewable Energy Laboratory.
42

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Appendix A: TRL Guideline Table

Technology Readiness Levels for Advanced Technology Bus Commercialization

Relative Level 

of Technology 

Development

Technology 

Readiness 

Level

TRL Definition Description

Deployment TRL 9

Actual system 

operated over the full 

range of expected 

conditions

The technology is in its final form. 

Deployment, marketing, and support begin for 

the first fully commercial products.

Technology 

Demonstration/ 

Commissioning

TRL 8

Actual system 

completed and 

qualified through test 

and demonstration

The last step in true system development. 

Demonstration of a limited production of 50 to 

100 buses at a small number of locations. 

Beginning the transition of all maintenance to 

transit staff.

TRL 7 Full-scale validation in 

relevant environment

A major step up from TRL 6 by adding larger 

numbers of buses and increasing the hours of 

service. Full-scale demonstration and 

reliability testing of 5 to 10 buses at several 

locations. Manufacturers begin to train larger 

numbers of transit staff in operation and 

maintenance.

TRL 6

Engineering/pilot-scale 

validation in relevant 

environment

First tests of prototype buses in actual transit 

service. Field testing and design shakedown 

of one to two prototypes. Manufacturers assist 

in operation and typically handle all 

maintenance. Begin to introduce transit staff to 

technology.

Technology 

Development

TRL 5

Laboratory scale, 

similar system 

validation in relevant 

environment

Integrated system is tested in a laboratory 

under simulated conditions based on early 

modeling. System is integrated into an early 

prototype or mule platform for some on-road 

testing.

TRL 4

Component and 

system validation in 

laboratory environment

Basic technological components are 

integrated into the system and begin 

laboratory testing and modeling of potential 

duty cycles.

Research to 

Prove 

Feasibility

TRL 3

Analytical and 

experimental critical 

function and/or proof of 

concept

Active research into components and system 

integration needs. Investigate what 

requirements might be met with existing 

commercial components.

TRL 2

Technology concept 

and/or application 

formulated

Research technology needed to meet market 

requirements. Define strategy for moving 

through development stages. Basic 

Technology 

Research TRL 1 Basic principles 

observed and reported

Scientific research and early development of 

concepts. 
43

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Appendix B: Foothill Fleet Summary Statistics

BEB and CNG Fleet Operations and Economics

BEB CNG

Number of vehicles 12 8

Period used for fuel and energy op analysis 4/14–7/15 10/14–7/15

Total number of months in period 16 10

Fuel and energy analysis base fleet mileage 401,244 294,533

Period used for maintenance op analysis 1/15–7/15 10/14–7/15

Total number of months in period 7 10

Maintenance analysis base fleet mileage 401,244 364,373

Average monthly mileage per vehicle 2,333 4,555

Availability 90 94

Fleet energy usage in kWh (BEB) or GGE (CNG) 864,586 72,979

Roadcalls 43 8

Total MBRC 9,331 45,547

Propulsion-related roadcalls 16 4

Propulsion-related MBRC 25,078 91,093

Fleet kWh per mile (BEB) or miles per GGE (CNG) 2.15 4.04

Representative fleet miles per DGE 17.48 4.51

Electricity cost per kWh or CNG cost per GGE 0.18 0.93

Energy cost per mile 0.39 0.23

Total scheduled repair cost per mile 0.08 0.14

Total unscheduled repair cost per mile 0.09 0.04

Total maintenance cost per mile 0.16 0.18

Total operating cost per mile 0.55 0.41

Maintenance Costs

BEB CNG

Fleet mileage 205,041 364,373

Total parts cost 9,344.72 32,761.73

Total labor hours 483.4 679.5

Average labor cost (@ $50.00 per hour) 24,170.00 33,972.50

Total maintenance cost 33,514.72 66,734.23

Total maintenance cost per bus 2,792.89 8,341.78

Total maintenance cost per mile 0.16 0.18
44

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Breakdown of Maintenance Costs by Vehicle System

BEB CNG

Fleet mileage 205,041 364,373

Total Engine/Fuel-Related Systems (ATA VMRS 27, 30, 31, 32, 33, 41, 42, 43, 

44, 45, 46, 65)

Parts cost 2,421.01 23,240.22

Labor hours 37.00 94.75

Average labor cost 1,850.00 4,737.50

Total cost (for system) 4,271.01 27,977.72

Total cost (for system) per bus 266.94 1,748.61

Total cost (for system) per mile 0.02 0.08

Exhaust System Repairs (ATA VMRS 43)

Parts cost 0.00 0.00

Labor hours 0.0 0.0

Average labor cost 0.00 0.00

Total cost (for system) 0.00 0.00

Total cost (for system) per bus 0.00 0.00

Total cost (for system) per mile 0.00 0.00

Fuel System Repairs (ATA VMRS 44)

Parts cost 0.00 64.68

Labor hours 0.0 6.5

Average labor cost 0.00 325.00

Total cost (for system) 0.00 389.68

Total cost (for system) per bus 0.00 24.36

Total cost (for system) per mile 0.00 0.00

Power Plant (Engine) Repairs (ATA VMRS 45)

Parts cost 0.00 15,590.27

Labor hours 10.5 25.5

Average labor cost 525.00 1,275.00

Total cost (for system) 525.00 16,865.27

Total cost (for system) per bus 32.81 1,054.08

Total cost (for system) per mile 0.00 0.05

Electric Propulsion Repairs (ATA VMRS 46)

Parts cost 0.00 0.00

Labor hours 1.5 0.0

Average labor cost 75.00 0.00

Total cost (for system) 75.00 0.00

Total cost (for system) per bus 4.69 0.00

Total cost (for system) per mile 0.00 0.00
45

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Breakdown of Maintenance Costs by Vehicle System (continued)

BEB CNG

Electrical System Repairs (ATA VMRS 30-Electrical General, 31-Charging, 32-

Cranking, 33-Ignition)

Parts cost 2,421.01 1,158.84

Labor hours 24.5 29.0

Average labor cost 1,225.00 1,450.00

Total cost (for system) 3,646.01 2,608.84

Total cost (for system) per bus 227.88 163.05

Total cost (for system) per mile 0.02 0.01

Air Intake System Repairs (ATA VMRS 41)

Parts cost 0.00 4,427.07

Labor hours 0.0 0.8

Average labor cost 0.00 37.50

Total cost (for system) 0.00 4,464.57

Total cost (for system) per bus 0.00 279.04

Total cost (for system) per mile 0.00 0.01

Cooling System Repairs (ATA VMRS 42)

Parts cost 0.00 1,810.22

Labor hours 0.0 24.5

Average labor cost 0.00 1,225.00

Total cost (for system) 0.00 3,035.22

Total cost (for system) per bus 0.00 189.70

Total cost (for system) per mile 0.00 0.01

Hydraulic System Repairs (ATA VMRS 65)

Parts cost 0.00 0.00

Labor hours 0.0 3.5

Average labor cost 0.00 175.00

Total cost (for system) 0.00 175.00

Total cost (for system) per bus 0.00 10.94

Total cost (for system) per mile 0.00 0.00

General Air System Repairs (ATA VMRS 10)

Parts cost 0.00 0.00

Labor hours 10.3 8.0

Average labor cost 512.50 400.00

Total cost (for system) 512.50 400.00

Total cost (for system) per bus 32.03 25.00

Total cost (for system) per mile 0.00 0.00
46

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Breakdown of Maintenance Costs by Vehicle System (continued)

BEB CNG

Brake System Repairs (ATA VMRS 13)

Parts cost 0.00 0.00

Labor hours 0.0 0.0

Average labor cost 0.00 0.00

Total cost (for system) 0.00 0.00

Total cost (for system) per bus 0.00 0.00

Total cost (for system) per mile 0.00 0.00

Transmission Repairs (ATA VMRS 27)

Parts cost 0.00 189.15

Labor hours 0.5 5.0

Average labor cost 25.00 250.00

Total cost (for system) 25.00 439.15

Total cost (for system) per bus 1.56 27.45

Total cost (for system) per mile 0.00 0.00

Inspections Only - no parts replacements (101)

Parts cost 0.00 0.00

Labor hours 295.2 438.5

Average labor cost 14,760.00 21,925.00

Total cost (for system) 14,760.00 21,925.00

Total cost (for system) per bus 922.50 1,370.31

Total cost (for system) per mile 0.07 0.06

Cab, Body, and Accessories Systems Repairs (ATA VMRS 02-Cab and Sheet 

Metal, 50-Accessories, 71-Body)

Parts cost 187.36 936.07

Labor hours 112.0 112.3

Average labor cost 5,597.50 5,615.00

Total cost (for system) 5,784.86 6,551.07

Total cost (for system) per bus 361.55 409.44

Total cost (for system) per mile 0.03 0.02

HVAC System Repairs (ATA VMRS 01)

Parts cost 0.00 887.58

Labor hours 0.5 11.4

Average labor cost 25.00 570.00

Total cost (for system) 25.00 1,457.58

Total cost (for system) per bus 1.56 91.10

Total cost (for system) per mile 0.00 0.00
47

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Breakdown of Maintenance Costs by Vehicle System (continued)

BEB CNG

Lighting System Repairs (ATA VMRS 34)

Parts cost 0.00 0.00

Labor hours 1.0 0.0

Average labor cost 50.00 0.00

Total cost (for system) 50.00 0.00

Total cost (for system) per bus 3.13 0.00

Total cost (for system) per mile 0.00 0.00

Frame, Steering, and Suspension Repairs (ATA VMRS 14-Frame, 15-Steering, 

16-Suspension)

Parts cost 0.00 34.72

Labor hours 0.0 4.0

Average labor cost 0.00 200.00

Total cost (for system) 0.00 234.72

Total cost (for system) per bus 0.00 14.67

Total cost (for system) per mile 0.00 0.00

Axle, Wheel, and Drive Shaft Repairs (ATA VMRS 11-Front Axle, 18-Wheels, 

22-Rear Axle, 24-Drive Shaft)

Parts cost 0.00 6,102.94

Labor hours 0.0 2.0

Average labor cost 0.00 100.00

Total cost (for system) 0.00 6,202.94

Total cost (for system) per bus 0.00 387.68

Total cost (for system) per mile 0.00 0.02

Tire Repairs (ATA VMRS 17)

Parts cost 6,736.35 1,560.20

Labor hours 27.5 8.5

Average labor cost 1,375.00 425.00

Total cost (for system) 8,111.35 1,985.20

Total cost (for system) per bus 506.96 124.08

Total cost (for system) per mile 0.04 0.01
48

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Notes 

1. To compare the charging energy to CNG fuel and diesel equivalent, the energy and CNG were

converted into diesel energy equivalent gallons. Actual energy content will vary by location, but the 

general energy conversions are based on the following: 

Lower heating value (LHV) for diesel = 128,488 Btu/gal (Alternative Fuels Data Center, fuel 

properties database8)

U.S. average energy content of electricity = 3,412 Btu/kWh (Energy Information Administration)

Conversion factor for calculations = 37.7 kWh/gal

CNG fuel is reported as gasoline gallon equivalent (GGE). The gasoline LHV is 115,000 Btu/gal.

Gasoline/Diesel = 115,000 Btu/gallon / 128,400 Btu/gallon = 0.896 GGE/gal

2. The propulsion-related systems were chosen to include only those systems of the vehicles that could 

be affected directly by the selection of a fuel or advanced technology. 

3. ATA VMRS coding is based on parts that were replaced. If there was no part replaced in a given repair, 

then the code was chosen by the system being worked on.

4. In general, inspections (with no part replacements) were included only in the overall totals (not by 

system). Category 101 was created to track labor costs for PMIs.

5. ATA VMRS 02-Cab and Sheet Metal represents seats, doors, etc.; ATA VMRS 50-Accessories 

represents things like fire extinguishers, test kits, fareboxes, etc.; ATA VMRS 71-Body represents mostly 

windows and windshields.

6. Average labor cost is assumed to be $50 per hour.

7. Warranty costs are not included.

 8 http://www.afdc.energy.gov/fuels/fuel_properties.php
49

This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Appendix C: Fleet Summary Statistics—SI Units

BEB and CNG Fleet Operations and Economics

BEB CNG

Number of vehicles 12 8

Period used for fuel and energy op analysis 4/14–7/15 10/14–7/15

Total number of months in period 16 10

Fuel and energy analysis base fleet mileage (km) 645,722 473,992

Period used for maintenance op analysis 1/15–7/15 10/14–7/15

Total number of months in period 7 10

Maintenance analysis base fleet kilometers 645,722 586,385

Average monthly kilometers per vehicle 3,754 7,330

Availability 90 94

Fleet fuel usage in kWh or CNG liter equiv. 864,586.0 276,255.6

Roadcalls 43 8

Total KMBRC 15,017 73,298

Propulsion roadcalls 16 4

Propulsion KMBRC 40,358 146,596

Rep. fleet fuel consumption (L/100 km) 13.44 52.16

Number of vehicles 12 8

Electricity cost per kWh or CNG cost per liter 0.18 0.25

Energy cost per kilometer 0.24 0.14

Total scheduled repair cost per kilometer 0.05 0.09

Total unscheduled repair cost per kilometer 0.05 0.03

Total maintenance cost per kilometer 0.10 0.11

Total operating cost per kilometer 0.34 0.26

Maintenance Costs

BEB CNG

Fleet mileage (km) 329,972 586,385

Total parts cost 9,344.72 32,761.73

Total labor hours 483.40 679.45

Average labor cost (@ $50.00 per hour) 24,170.00 33,972.50

Total maintenance cost 33,514.72 66,734.23

Total maintenance cost per bus 2,792.89 8,341.78

Total maintenance cost per kilometer 0.10 0.11