Calculating Generator Reliability

Generators can fail due to:

• Failure to Start (FTS)

• Failure to Run (FTR)

• Failure of Fuel Supply (FFS)

The likelihood of each failure type varies with outage length 

and fuel supply. 

Many Power outages are short duration, but some can last 

for several days. 

When calculating reliability, the distribution of outage 

durations, and the type of fuel used need to be considered.

The information contained in this poster is subject to a government license.

JISEA Annual Meeting, Golden, Colorado

March 13 – 14, 2019

NREL/PO-6A50-73462

A Comparison of Fuel Choice for Backup Generators

Sean Ericson and Dan Olis

Joint Institute for Strategic Energy Analysis (JISEA)

This work was authored by the National Renewable Energy Laboratory, operated by Alliance for Sustainable Energy, LLC, for the U.S. Department of Energy (DOE) under Contract No. DE-AC36-08GO28308. Funding provided 

by Enchanted Rock, LLC. The views expressed herein do not necessarily represent the views of the sponsor, DOE, or the U.S. Government. The U.S. Government retains and the publisher, by accepting the article for 

publication, acknowledges that the U.S. Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for U.S. Government 

purposes.

Goals Of Project

• Estimate the reliability of diesel and natural gas backup 

generators.

• Compare how differences in fuel security between diesel 

and natural gas affects backup generator reliability for 

long-duration outages.

• Model the economics of using backup generators to 

provide additional services.

• Compare the economics of diesel and natural gas backup 

generators.

Customer outage-hour distributions by region. 

Failure Type Point Estimate Lower Range Upper Range

FTS (Backup Only) 0.67% 0.33% 1.34%

FTS (Grid Services) 0.15% 0.08% 0.3%

FTR/MTTF (Backup Only) 0.172% / 580 hr 0.086% / 1156 hr 0.346% / 279 hr

FTR/MTTF (Grid Services) 0.086% / 1160 hr 0.043% / 2312 hr 0.173% / 578 hr

Diesel Fuel Supply 36 hr 18 hr 72 hr

Failure of Resupply (Diesel) 14% 7% 28%

FFS (NG Long-Duration) 1.5% 0.75% 3%

Map of Restructured markets in the United States

Likelihood of backup surviving outage by fuel type and outage duration.

Additional Customer Services

In addition to reliability benefits, backup generators may:

• Allow a facility to switch to an interruptible rate tariff.

• Reduce energy and demand charges.

• In restructured markets generators can sell into wholesale markets.

• Reduce coincident peak charges.

More frequent operations also reduces failure likelihoods. Generator Type Diesel Natural Gas

Region TX FL NJ TX FL NJ

CAPEX + O&M ($/kW) -$1,205 -$1,405

Revenues/savings ($kW) $968 $1,380 $3,064 $1,091 $1,380 $3,153

Fuel cost for ($/kW) -$187 $0 -$341 -$199 $0 -$272

NPV ($/kW) -$425 $175 $1,518 -$513 $25 $1,476

Case Studies

• We gathered outage data, cost data, rate tariffs, generator 

reliability data, and reliability of fuel supply data from a 

variety of sources. 

• We compare the reliability and economic benefits of a 

backup diesel system with 36 hours of on-site fuel and a 

natural gas system installed in Florida, Texas, and New 

Jersey, to provide backup power and grid services to a 

supermarket.

Failure rate estimates for various backup generator configurations

Region Diesel Reliability (%) Natural Gas Reliability (%) Difference 

U.S. Average 94.7 97.3 2.6

Florida 90.1 95.5 5.4

New Jersey 97.2 98.2 1.0

Texas 97.3 98.3 1.0

Main Findings

• Due to lower rates of fuel supply failure during long duration 

outages, natural gas generators can provide higher reliability 

than diesel generators.

• Generators which provide grid services can significantly 

reduce net costs, or even provide net positive revenues.

• Due to lower capital costs, diesel generators tend to be 

cheaper than natural gas generators, even after accounting 

for the lower fuel costs of natural gas

• Differences between fuel types are relatively modest. 

Additional considerations may determine investment choice.

Reliability estimates by fuel type and region

Net Present Values by region and by fuel type