About

kWh Math is an independent calculator library for US households comparing electricity cost, solar, EV charging, batteries, appliances, and heating fuels. We publish the arithmetic, assumptions, worked examples, and source vintage beside each result so an estimate can be checked rather than merely accepted. The site is informational: it does not sell leads, does not broker quotes, and does not replace a utility tariff, an engineering design, tax advice, or a contractor proposal.

Data method and vintages

State electricity prices are rounded from the U.S. Energy Information Administration Electric Power Monthly, Table 5.6.A, 2024 annual residential average revenue per kilowatt-hour. Typical monthly consumption is rounded from Form EIA-861 residential sales and customer counts for 2023. Solar production factors are planning values informed by NREL PVWatts Version 8 conventions for a fixed, generally south-facing array carrying typical system losses. City coordinates are rounded offline geographic centroids, and appliance wattages are planning presets drawn from Department of Energy Energy Saver guidance.

Those vintages are stated on every page that uses them, in body copy and again in the Sources block, because the underlying tables are revised annually. A page title may carry the current year where the data is refreshed on an annual cycle, but the body never claims a rate is newer than the table it came from. When a vintage changes we update the bundled data, the visible source notes, and the known-answer tests in the same change. Every benchmark in every tool is an editable field, so your own tariff, meter reading, quote, or equipment specification always wins over ours.

How the calculators are built and checked

Each calculation lives in a plain function with unit tests covering hand-worked cases, zero values, and invalid inputs, and each page is rendered in an automated test that checks its heading, metadata, structured data, and canonical URL before the site is built. Explanations are written to keep the distinctions that matter: energy against power, useful delivered heat against fuel input, nameplate solar capacity against modeled AC production, and the marginal energy rate against an all-in average that has fixed charges baked into it.

Calculator inputs are stored in the page URL rather than in an account, so a scenario can be bookmarked, reopened, or sent to an installer without signing in. The arithmetic itself runs entirely in your browser. We use Microsoft Clarity for aggregate usage analytics to see which tools and explanations are actually used; the privacy page describes what that involves and what it does not collect.

Reporting an error

Corrections are welcome and are treated as the fastest way to improve the site. Use the contact page and include the page URL with its query string, the inputs you used, the result you expected, and the formula or published source you checked against. Reports about a stale official table, a broken citation, a keyboard or screen-reader problem, or a layout failure on a small screen are equally useful. We publish a correction rather than quietly editing a number when a result was materially wrong.