Behind-the-Meter Solar + Storage 

Modeling Tool Comparison

Kathleen Krah, NREL

ASES Solar 2018 – Boulder, CO

August 2, 2018
NREL | 2

Background / Motivation

Tool Overview

Tool Capabilities and Considerations

Tool Comparison

Big Picture: Energy Modeling of BTM solar + storage
NREL | 3

Introduction

Kathleen Krah

• Engineer, NREL’s Integrated Application Center – Modeling & Analysis Team

– Focus on techno-economic modeling and 

optimization of distributed energy resources 

for cost savings, clean energy, and resiliency 

targets

• BS Chemical & Biomolecular Engineering, 

University of Notre Dame

• MEngSc Sustainable Energy Engineering, 

University College Cork (Ireland)
NREL | 4

Motivation

We’ve been receiving – and asking 

ourselves – many questions about: 

• What tools should we use for 

different analyses?

• What assumptions are being 

made by the tools we’re using, 

and how accurate are they to 

the actual system we’re 

modeling?

• Where do our tools fit into the 

suite of tools available?

• What are the most important 

tool development and future 

modeling needs?
NREL | 5

REopt Lite

NREL

https://reopt.nrel.gov/tool

System Advisor Model (SAM)

NREL

https://sam.nrel.gov/

HOMER Grid

HOMER Energy

https://www.homerenergy.com/products/gri

d/index.html

Behind-the-meter PV + storage 

technoeconomic modeling tools 

Distributed Energy Resources –

Customer Adoption Model 

(DER-CAM)

LBNL

https://buildingmicrogrid.lbl.gov/projects/der-cam

ESyst

Geli

https://esyst.geli.net/

Storage Value Estimate Tool 

(StorageVET)

EPRI

https://storagevet.com/
NREL | 6

SOLAR PV MODEL

RESOURCE, 

PERFORMANCE

BATTERY MODEL

PERFORMANCE, 

DEGRADATION, 

REPLACEMENT

UTILITIES

RATES, NET METERING, 

COST ESCALATION, GRID 

SERVICES

LOADS

SIMULATION, 

CUSTOMIZATION, LOAD 

GROWTH

ECONOMICS

COSTS, OWNERSHIP, 

INCENTIVES, 

DEPRECIATION, TAX

RESILIENCY

OUTAGES, CRITICAL 

LOADS, BACKUP 

GENERATORS

OPTIMIZATION / 

SIMULATION

SIZING, DISPATCH, 

FORECASTING, TIME 

RESOLUTION

USER INTERFACE

INPUTS, OUTPUTS, 

USABILITY, COMPLEXITY

OTHER

EMISSIONS, POWER 

FLOW, SINGLE/MULTINODAL, SENSITIVITY 

STUDIES, OTHER 

TECHNOLOGIES

Model Capabilities to Consider
NREL | 7

SOLAR PV MODEL

RESOURCE, 

PERFORMANCE

BATTERY MODEL

PERFORMANCE, 

DEGRADATION, 

REPLACEMENT

UTILITIES

RATES, NET METERING, 

COST ESCALATION, GRID 

SERVICES

LOADS

SIMULATION, 

CUSTOMIZATION, LOAD 

GROWTH

ECONOMICS

COSTS, OWNERSHIP, 

INCENTIVES, 

DEPRECIATION, TAX

RESILIENCY

OUTAGES, CRITICAL 

LOADS, BACKUP 

GENERATORS

OPTIMIZATION / 

SIMULATION

SIZING, DISPATCH, 

FORECASTING, TIME 

RESOLUTION

USER INTERFACE

INPUTS, OUTPUTS, 

USABILITY, COMPLEXITY

OTHER

EMISSIONS, POWER 

FLOW, SINGLE/MULTINODAL, SENSITIVITY 

STUDIES, OTHER 

TECHNOLOGIES

Model Capabilities to Consider
NREL | 8

•Solar resource

-NSRDB tmy2/tmy3, NASA, others

-custom

•PV performance

-PV output profile

-simple efficiency

-NREL PVWatts1

-specific product lines from 

manufacturers

-voltage/current tracking

-temperature effects

-losses (%, detailed)

-shading (%, 3D model)

-degradation

-separate inverter model (DC-to-AC 

ratio, detailed)

•Existing PV

SOLAR PV MODELS

SOLAR PV MODELS

1 https://pvwatts.nrel.gov/pvwatts.php

Tool SOLAR PV MODELS

REopt Lite User enters location, tool calls PVWatts using NSRDB tmy3 data. Considers existing PV.

SAM Two options: 1) detailed PV model- NSRDB or custom resource data; 3D shading model 

and detailed losses; tracks PV module and inverter current, voltage, temperature effects, 

and degradation (% annual degradation); applies actual manufacturer PV and inverter 

specs. 2) PVWatts model- NSRDB or custom resource data; tool calls PVWatts

HOMER Grid Pulls solar resource data from NREL or NASA database; HOMER synthesizes solar radiation 

using V.A. Graham algorithm for daily/hourly variability applied to downloaded or userdefined monthly averages. Generation calculated using equation considering generic or 

specific PV cell characteristics, incident solar radiation, PV cell temp, and derating factor 

DER-CAM Custom solar resource data. how do they calculate PV output? Inverters modeled as 

piecewise linear function of power in and power out- DC to AC ratio?? Considers existing 

PV.

ESyst User uploads PVWatts, HelioScope, or PVSyst results or generic table of generation; annual 

% degradation; considers time-based inverter replacement

StorageVET
NREL | 9

•Battery performance

-simple efficiency

-voltage/current tracking

-temperature effects on capacity 

and lifetime

-battery chemistries

-specific battery product lines from 

manufacturers

-maximum charge/discharge rates

-strings/paralleling of batteries

-ancillary equipment losses

•Degradation

-calendar degradation

-cycling degradation

-replacement

BATTERY MODELS

BATTERY MODELS

Tool BATTERY MODELS

REopt Lite Simple roundtrip efficiency with time-based replacement

SAM Two options: 1) detailed PV model- considers current and voltage measurements and 

constraints; temperature effects, operating point on I-V curve, maximum charge/discharge 

rates; different battery chemistries for voltage models and degradation; cycling and 

calendar degradation with option to replace based on either; applies specific 

manufacturer specs. 2) PVWatts model- simple roundtrip efficiency and % degradation, 

doesn’t track current, voltage, temperature effects, operating point, etc.

HOMER Grid Three models: simple battery model, kinetic battery model, modified kinetic battery 

model (option to consider temperature effects on capacity and lifetime); considers voltage 

and maximum charge/discharge power, and batteries in strings and in parallel; user 

selects generic or specific batteries; cycling and calendar degradation with option to 

replace based on either or both

DER-CAM Considers efficiency, self-discharge, temperature effects?, user-specified maximum 

charge/discharge power. Applies cycling degradation model for Li-ion batteries to 

calculate capacity.replacement? AccurateBattChargeDischarge?

ESyst Simple roundtrip efficiency (user-specified or from specific battery manufacturer info); 

User-specified calendar degradation rate + internally calculated cycling degradation model 

(based on information directly from battery manufacturers); replace when effective 

capacity reaches 80% or in calendar-based replacement year, whichever occurs first

StorageVET
NREL | 10

•Sizing

•Dispatch

- peak shaving / demand charge reduction

- energy arbitrage

- load control

•Forecasting / lookahead

•Maximum technical potential vs expected 

savings

•Chronology and resolution of data and analysis

•Objective function

- costs (lifecycle costs, NPV, electricity costs)

- emissions

•Type of optimization (MILP, grid search, 

simulation only)

OPTIMIZATION / SIMULATION

•Outages (frequency, duration, 

planned/unplanned)

•Critical loads (% of full load, custom load, 

prioritization of load curtailment, value of lost 

loads)

•Backup generators (fuel rates, minimum load, 

fuel availability)

RESILIENCY

OPTIMIZATION / SIMULATION, RESILIENCY

Tool OPTIMIZATION / SIMULATION RESILIENCY

REopt Lite MILP; optimizes PV and battery storage sizes to maximize system NPV 

using energy arbitrage and demand charge reduction; assumes perfect 

forecasting for maximum technical potential; optimization performed on 

one year of chronological annualized data applied to project lifetime

✔

SAM Simulation only, parametrics for pseudo-optimization; day-ahead or daybehind peak-shaving or custom dispatch strategy- currently minimizes grid 

purchases, but in future will consider utility rate

---

HOMER Grid Grid search or proprietary non-derivative optimization; currently models 1 

year of chronological timesteps- planned implementation of multi-year 

analysis in Q3 of 2018.

✔

DER-CAM ✔

ESyst Maximize savings on the utility bill based on interval data, PV data, and 

utility tariff using same algorithms used in run-time controls 

(forecasting/optimization/dispatch). User selects between optimal 

(theoretical maximum) savings, estimated savings (more realistic based on 

Geli load forecaster and demand charge management), and conservative 

savings (like estimated, but with a more conservative demand charge 

management)

---

StorageVET ---
NREL | 11

•Shape

- DOE commercial reference buildings2

- other generic load profiles

- custom load input

- outputs from UtilityAPI3, Green Button4, etc

•Magnitude

- scaling to monthly or annual totals

•Load growth

LOADS

•Utility rate

- database (URDB5, Genability6)

- custom utility rate inputs

- energy and demand charges; TOU, tiered, realtime, seasonal, weekend/ weekday/holiday etc.

- post-solar rate study

•Net metering

- net metering limit

- export limitations

- true net metering vs avoided cost payments

- net metering accounting

•Energy cost escalation rates

UTILITIES

LOADS, UTILITIES, ECONOMICS, 

RESILIENCY, USER EXPERIENCE, OTHER

•Costs considered (capital, O&M)

•Ownership models (direct purchase, 3rd party/PPA, 

etc.)

•Financial parameters considered (discount rates, 

inflation, cost escalation rates)

•Incentives

- capital-based (%), such as federal ITC

- capacity-based ($/kW)

- production-based ($/kWh)

- federal, state, utility/local

- depreciation (straight line, MACRS, bonus MACRS)

•Tax models

- income, property, sales, etc.

- Interplay between tax and incentives

ECONOMICS

•Outages (frequency, duration, planned/unplanned)

•Critical loads

- % of full load

- custom load

- prioritization of loads for curtailment

- value of lost loads

•Backup generators

- fuel rates

- minimum load

- fuel availability

RESILIENCY

•Access, cost

•User account

•Inputs (user expertise and data 

requirements)

•data requirements and level of detail and 

expertise desired for stage and emphasis 

of analysis

•tradeoff between simplicity vs 

customizability

•Outputs (content and format)

•system sizing

•dispatch

•maximum technical potential vs expected 

savings

•proforma (interactive with formulas vs 

static)

USER EXPERIENCE

•Other technologies

•Emissions modeling

•Power flow modeling

•Single or multi-nodal analyses

•Sensitivity study capabilities

OTHER

2 https://www.energy.gov/eere/buildings/commercial-reference-buildings; 3 https://utilityapi.com/; 4 http://www.greenbuttondata.org/; 5 https://openei.org/wiki/Utility_Rate_Database; 6 https://www.genability.com/
www.nrel.gov

Thank you!

kathleen.krah@nrel.gov

NREL/PR-7A40-71804

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 U.S. 

Department of Energy Office of Energy Efficiency and Renewable Energy Federal Energy Management Program. The 

views expressed in the article do not necessarily represent the views of the 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.
NREL | 13

Tool Overview

Tool REopt Lite System Advisor 

Model (SAM)

HOMER Grid DER-CAM ESyst StorageVET

Organization NREL NREL HOMER Energy Desktop Web Web

Type of 

application

Web Desktop; Software 

development kit; Open 

source code

Desktop Web and desktop Web Web

Cost/Access Free, Publicly 

available

Free; Open source For purchase Free, Publicly available Free, Publicly 

available

Free, Publicly 

available

Optimization/

Simulation 

MILP Simulation; Parametric for 

pseudo-optimization

- Enhanced grid search

- Proprietary 

derivative-free 

optimization

MILP Simulation Simulation

Technologies PV, battery, 

diesel generator, 

existing PV

PV, high concentration PV, 

wind, biomass, 

geothermal, solar hot 

water, CSP, process heat 

PV, wind, CHP, boiler, 

electric heater, diesel 

generator, battery

PV, solar thermal, wind, hydro, 

CHP, energy storage (battery, 

EVs, thermal), controllable 

loads, HVAC

PV, battery PV, wind, battery

Technology 

location(s)

Behind-themeter; Resiliency 

(outages)

Behind-the-meter Behind-the-meter; 

Resiliency (outages)

Behind-the-meter; Resiliency 

(outages)

Behind-themeterBehind-themeter, utilityscale
NREL | 14

•Shape

- DOE commercial reference buildings2

- other generic load profiles

- custom load input

- outputs from UtilityAPI3, Green Button4, etc

•Magnitude

- scaling to monthly or annual totals

•Load growth

LOADS

•Utility rate

- database (URDB5, Genability6)

- custom utility rate inputs

- energy and demand charges; TOU, tiered, realtime, seasonal, weekend/ weekday/holiday etc.

- post-solar rate study

•Net metering

- net metering limit

- export limitations

- true net metering vs avoided cost payments

- net metering accounting

•Energy cost escalation rates

UTILITIES

LOADS, UTILITIES, ECONOMICS

Tool LOADS UTILITIES

REopt Lite DOE commercial reference buildings, custom, 

critical load

URDB database; custom rate planned for end of 

2018. True net metering option up to net 

metering limit or annual load. 

SAM DOE commercial reference buildings, custom URDB database, custom rates; several net 

metering options

HOMER Grid DOE commercial reference buildings; generic 

commercial, industrial, residential, community 

loads; custom; critical load 

Genability database for commercial North 

American tariffs; detailed custom tariff builder. 

Net metering based on rate. User can also set a 

kW grid sale limit.

DER-CAM Typical weekday, weekend, and “peak day” load 

for each month; load database based on 

ASHRAE regions- DOE commercial reference 

buildings?

Limited options for representative existing rates 

in select cities. Also custom rate input optionTOU, tiered, or hourly energy charges, 

coincident, non-coincident, and TOU daily and 

monthly demand charges.

ESyst Custom load, incl. UtilityAPI and Green Button 

outputs

Genability database for commercial North 

American tariffs; custom rate planned for 

upcoming release. Net metering based on rate, 

with NEM and NEM2.0 for CA rates. User can 

select separate post-solar utility rate.

StorageVET

2 https://www.energy.gov/eere/buildings/commercial-reference-buildings; 3 https://utilityapi.com/; 4 http://www.greenbuttondata.org/; 5 https://openei.org/wiki/Utility_Rate_Database; 6 https://www.genability.com/
NREL | 15

•Costs considered (capital, O&M)

•Ownership models (direct purchase, 3rd

party/PPA, etc.)

•Financial parameters considered (discount rates, 

inflation, cost escalation rates)

•Incentives

- capital-based (%), such as federal ITC

- capacity-based ($/kW)

- production-based ($/kWh)

- federal, state, utility/local

- depreciation (straight line, MACRS, bonus 

MACRS)

•Tax models

- income, property, sales, etc.

- Interplay between tax and incentives

ECONOMICS

•Outages (frequency, duration, 

planned/unplanned)

•Critical loads

- % of full load

- custom load

- prioritization of loads for curtailment

- value of lost loads

•Backup generators

- fuel rates

- minimum load

- fuel availability

RESILIENCY

ECONOMICS & RESILIENCY

Ownership 

model Tax modelsIncentives Depreciation Resiliency

REopt Lite Direct One overall 

rate

PV: federal, state, utilitycapital/capacity-based; total 

production-based. Battery: federal % 

capital

PV, storage: 5-yr, 7-yr 

MACRS, none

Recommends system sizes to 

meet critical load during 

outage(s); outage modeled as 

annual or once per system 

lifetime; considers existing 

backup generator

SAM

Direct; 3rd

party/PPA; 

others

Sales,

Overall: federal and state ITC, PTC; 

federal, state, utility, other direct 

cash, capacity-based, and 

production-based incentives- user 

select whether taxable and whether 

reduces depreciation and ITC bases

Overall: 5-yr, 7-yr 

MACRS, straight line, 

custom

---

Homer Grid Direct

Individual 

marginal tax 

rate applied 

to each 

incentive

PV, wind, storage: ITC; SGIP; custom 

capital/capacity-based and 

production-based; user selects 

portion eligible for each incentive

PV, wind, storage: 5-yr, 

7-yr MACRS, bonus 

MACRS, straight line, 

custom, none

User-defined outage duration 

and frequency- for outage 

frequency, models with and 

without outage and calculates 

weighted average of LCC

DER-CAM Direct ---

ESyst Direct

Federal and 

state income 

tax; state and 

city/county 

sales tax; 

utility tax

PV, overall: ITC. Overall: custom 

direct cash, capacity-based, 

production-based

Overall: federal and 

state 5-yr, 7-yr MACRS, 

100% bonus MACRS, 

straight line

---

StorageVET Direct
NREL | 16

•Access, cost

•User account

•Inputs (user expertise and data 

requirements)

•data requirements and level of detail 

and expertise desired for stage and 

emphasis of analysis

•tradeoff between simplicity vs 

customizability

•Outputs (content and format)

•system sizing

•dispatch

•maximum technical potential vs 

expected savings

•proforma (interactive with formulas 

vs static)

USER EXPERIENCE

•Other technologies

•Emissions modeling

•Power flow modeling

•Single or multi-nodal analyses

•Sensitivity study capabilities

OTHER

USER EXPERIENCE & OTHER

Tool ACCESS OUTPUTS OTHER

REopt Lite Web interface, free. Interactive proforma and 

annual/lifecycle costs summary

REopt Lite is a simplified version of a highly 

customizable desktop model that includes other 

technologies, developed and used at NREL for 

optimization analyses

SAM Desktop interface 

with software 

development kit and 

open source code, 

free.

Interactive proforma and 

annual/lifecycle costs summary 

with detailed D-VIEW plots and 

CSVs of hourly, monthly, annual, 

lifecycle data

Sensitivity studies, P50/P90 analyses

Other technologies: high concentration PV, wind, 

biomass, geothermal, solar hot water, CSP, process 

heat 

HOMER Grid Desktop interface, 

paid license.

Interactive proforma and 

annual/lifecycle costs summary 

with detailed D-VIEW plots and 

CSVs of hourly, monthly, annual, 

lifecycle data

Other technologies: wind, CHP, boiler, electric 

heater, diesel generator

HOMER Energy’s other tool, HOMER Pro, focuses on 

off-grid systems, while HOMER Grid focuses on 

behind-the-meter systems

DER-CAM Web and desktop 

interfaces, free. User 

account saves 

models.

Outputs Multi-nodal; power flow modeling; emissions 

modeling; sensitivity studies

Other technologies: solar thermal, wind, hydro, CHP, 

EVs, thermal storage, controllable loads, HVAC

ESyst Web interface, free. 

User account saves 

models and results. 

Interactive proforma and PDF 

report with visuals illustrating 

costs/benefits

Geli maintains commercial relationships with system 

integrators and provides a path forward beyond 

ESyst for users looking for hardware quotes and 

deploying an energy storage project

StorageVET Web interface, free. 

User account with 

saved models.

XXX Other technologies: wind

XXX