Sources and Methods

The figures in the free check and the report are computed from the information you enter and from the published data listed on this page. The sources of the reference figures, and the dates the figures were obtained, are stated in the free check and the report.

Each provider below is listed with the credit and link it requires, its terms of use or license where it publishes them, and the date each stored table was obtained. Stored tables are reviewed by the date shown.

To report a figure that appears to be incorrect, email support@yoursolarmath.com.

National Laboratory of the Rockies (NLR), formerly the National Renewable Energy Laboratory (NREL)

Solar production estimates (PVWatts), utility rates for a ZIP code (Utility Rates API), residential energy use by hour and end use (ResStock), and cost and performance defaults (Annual Technology Baseline).

Credit: NREL PVWatts, NREL Utility Rates API, NREL ResStock and the NREL Annual Technology Baseline, U.S. Department of Energy. Their use does not imply endorsement by the U.S. Department of Energy or the laboratory. developer.nlr.gov

Terms: NLR developer network terms

PVWatts production and the utility rate for a ZIP code are requested when a check runs or a report is generated.

OpenEI U.S. Utility Rate Database

Fixed monthly charges on residential bills, and the utility rates provided through the NREL Utility Rates API.

Credit: U.S. Utility Rate Database, OpenEI. apps.openei.org

Terms: Creative Commons Zero (CC0), as stated on the database's site

U.S. Energy Information Administration (EIA)

Average residential electricity and fuel prices by state, utility customer counts (EIA-861), household energy use (Residential Energy Consumption Survey), and regional wholesale electricity prices (Short-Term Energy Outlook).

Credit: U.S. Energy Information Administration, with the publication and its date stated beside each figure. eia.gov

Terms: EIA copyright and reuse

EnergySage

The price benchmark: the average price per watt of quotes on the EnergySage marketplace, by state, and the national average where no state figure is published (Alaska).

Credit: EnergySage, average of quotes on its marketplace, by state, with the month of the quotes. energysage.com

Terms: EnergySage terms of use

EnergySage receives payment from installers that take part in its marketplace. The figures are not obtained from the EnergySage website automatically. The figures shown are those obtained on the date below and are not updated until EnergySage gives written permission.

Lawrence Berkeley National Laboratory, Tracking the Sun

Installed prices of residential systems by state. The figures are kept for reference and are not shown as a comparison, because they cover systems installed before the federal tax credit ended.

Credit: Tracking the Sun, Lawrence Berkeley National Laboratory, licensed under CC BY 4.0. emp.lbl.gov

Terms: CC BY 4.0

Rewiring America

State, utility and local incentive programs for a ZIP code, which the free check and the report list as programs to ask the installer about.

Credit: Program list from Rewiring America. rewiringamerica.org

Terms: Rewiring America API terms

The program list is requested when a check runs or a report is generated, and no copy is kept longer than 24 hours, except in the report delivered to the buyer.

DSIRE, NC Clean Energy Technology Center

A link to the state's program list when the Rewiring America list is not available.

Credit: Database of State Incentives for Renewables and Efficiency (DSIRE). programs.dsireusa.org

Linked only. No figures are obtained from DSIRE.

U.S. Environmental Protection Agency (EPA)

Grid emission rates by region (eGRID) and fuel emission factors (GHG Emission Factors Hub).

Credit: U.S. Environmental Protection Agency, eGRID and GHG Emission Factors Hub. Works of the U.S. government are not subject to copyright (17 U.S.C. 105). epa.gov

Our World in Data

Per-person greenhouse gas emissions, for scale.

Credit: Our World in Data, greenhouse gas emissions. ourworldindata.org

NOAA National Centers for Environmental Information

Climate normals 1991 to 2020: heating and cooling degree days, temperatures and snowfall near each ZIP code.

Credit: NOAA NCEI U.S. Climate Normals 1991-2020. Works of the U.S. government are not subject to copyright (17 U.S.C. 105). ncei.noaa.gov

U.S. Census Bureau

ZIP code locations, counties and populations.

Credit: U.S. Census Bureau, Gazetteer files and the 2020 Census. Works of the U.S. government are not subject to copyright (17 U.S.C. 105). census.gov

Federal Reserve Economic Data (FRED), Federal Reserve Bank of St. Louis

Treasury bill and 10-year Treasury yields (Federal Reserve Board H.15) and consumer price inflation (U.S. Bureau of Labor Statistics CPI-U), used for the required return and for cost growth.

Credit: Board of Governors of the Federal Reserve System and U.S. Bureau of Labor Statistics, retrieved from FRED, Federal Reserve Bank of St. Louis. fred.stlouisfed.org

Terms: FRED legal notices

Aswath Damodaran, NYU Stern School of Business

Long-run historical returns of the S&P 500 and of a 60/40 mix, used for the required return. S&P 500 is a trademark of S&P Dow Jones Indices, which is not affiliated with this product.

Credit: Aswath Damodaran, Historical Returns on Stocks, Bonds and Bills, NYU Stern. pages.stern.nyu.edu

Other U.S. government publications

Driving and vehicle efficiency defaults (Federal Highway Administration National Household Travel Survey, FuelEconomy.gov) and the long-term capital gains rate (Internal Revenue Service), as named beside each default.

Credit: Federal Highway Administration, FuelEconomy.gov (U.S. Department of Energy and EPA) and the Internal Revenue Service. Works of the U.S. government are not subject to copyright (17 U.S.C. 105). fueleconomy.gov

Northeast Energy Efficiency Partnerships (NEEP)

Heat pump efficiency and capacity at low outdoor temperatures.

Credit: NEEP Cold Climate Air Source Heat Pump Specification and Product List. ashp.neep.org

Pacific Gas and Electric Company, Southern California Edison and San Diego Gas & Electric

California export credit values under the net billing tariff, and the time-of-use retail rates each utility requires with it.

Credit: Tariffs filed by PG&E, SCE and SDG&E with the California Public Utilities Commission, as published by each utility. cpuc.ca.gov