How We Verify Every Calculator
Every default value and formula on HowMuchWatts is checked against a public, authoritative source — not guessed. Here's exactly where our numbers come from, so you can verify them yourself.
Our sources
Rather than inventing plausible-sounding numbers, we anchor every default to a named public source:
- Electricity rates — our default of $0.17/kWh is the U.S. Energy Information Administration (EIA)'s published average residential electricity price, hand-checked against the EIA figure. Always replace it with your own rate from a recent bill for an exact result.
- Solar system cost — the default install cost and per-kW production figures are anchored to the National Renewable Energy Laboratory (NREL) residential PV cost benchmark and panel manufacturer spec sheets — not a single arbitrary quote. Your own installer quote will always be the most accurate input.
- Heat pump efficiency (COP) — our default COP of 3.2 falls inside the ENERGY STAR / U.S. Department of Energy qualifying range (roughly 3.1–4.1 for certified systems; 2.0–5.0 for air-source heat pumps generally).
- Appliance wattages (fridges, dryers, ovens, etc.) — set to the typical range published on ENERGY STAR product listings and manufacturer spec sheets, not a single arbitrary guess.
- Physical constants — conversions like watts↔BTU (×3.412) and the specific heat of water (4.186 kJ/kg°C, used in our water-heating calculator) are standard physics constants, not estimates.
How our formulas work
Every calculator on this site shows its exact formula in a "How it works" section on the page itself — nothing is hidden in a black box. If a formula simplifies reality (for example, our solar payback calculator uses simple payback rather than modeling electricity-price inflation), we say so directly on that page, and explain which direction the simplification biases the result.
Our update process
The default electricity rate is pulled directly from the EIA's public API and refreshed on every site build, so it tracks the latest published national average automatically rather than drifting out of date. Other reference figures (heat pump COP ranges, appliance wattages, NREL solar costs) are reviewed on a quarterly cadence against the sources above. We do not silently change numbers — each calculator's "Last updated" date reflects the most recent real review, and the rate default carries its own EIA sync date shown above.
Who maintains this site
HowMuchWatts is written and maintained by our small team — see About. We don't publish under invented expert personas; every figure here is traceable to the public sources listed above, which you're welcome to check yourself.
Found an error?
If a default value, formula, or explanation looks wrong, tell us at hello@howmuchwatts.com and we'll review and correct it. Corrections are logged in that calculator's "Last updated" date.