Sources we use
We use primary sources only: U.S. Energy Information Administration data and publications (EIA-861, EIA-861M, Electric Power Monthly, Natural Gas Monthly, the State Heating Oil and Propane Program, Energy Explained); U.S. Department of Energy and ENERGY STAR guidance and federal efficiency standards (10 CFR Part 430); and state public utility commissions and their official shopping sites. Energy brokers, supplier marketing pages and comparison sites are never used as sources of fact.
How calculators are checked
Every calculator uses published, standard formulas. The code is covered by automated tests that check known answers (for example, a 900 kWh month at the U.S. average price, the propane and heat pump cost of 57 million Btu of heat, and the ENERGY STAR room chart adjustments are recomputed by hand in our automated tests). We show the formula next to each tool so anyone can verify the result by hand.
What we will not publish
- Affiliate links, paid plan rankings, supplier sign-up forms or lead forms for contractors.
- Prices from broker or supplier marketing pages presented as averages.
- Claims we cannot support with a source.
Updates and corrections
Each page shows the date it was last updated. EIA publishes electricity and natural gas data monthly (about two months after the data month) and heating oil and propane prices weekly from October through March. We refresh the data when EIA does, re-run the tests, and change page dates only when content changes. If you find an error, tell us through the contact page and we will review it and correct it.