Methodology
Every figure on this site is an estimate from inputs you can see and change. This page states the formulas, where the default numbers come from, how they are rounded, what is left out, and how often the data is checked.
Energy
Vehicle efficiency is entered battery-side, as kWh per 100 miles, which is what cars and EPA labels report. What the meter sees is more, because some energy is lost in the charger, cable and battery management. We call that the charging loss and default it to 12%.
Wall energy = miles × (kWh per 100 miles ÷ 100) ÷ (1 − loss). A month is 30.4 days.
Cost
- Home cost = wall kWh × share charged at home × home rate.
- Public cost = wall kWh × (1 − share at home) × public rate.
- Gas cost = miles ÷ mpg × price per gallon.
- Cost per mile = monthly cost ÷ monthly miles. Annual = monthly × 12 with no seasonal adjustment, and is labelled that way.
Money is kept in whole cents. Each cost component is rounded once, when kilowatt-hours meet a rate; totals are sums of those integers. Per-mile and percentage figures are derived from the totals and rounded only when displayed.
Charging time
Hours = battery kWh × (target − start) ÷ (charger kW × (1 − loss)). Constant power is assumed; real sessions taper above roughly 80%.
Default data
- Electricity by state: EIA Electric Power Monthly, Table 5.6.A (EIA-861M), average residential retail price, period 2026-08, retrieved 2026-11-02. Refreshed monthly; the site will not build if this file is older than 120 days.
- Gas: AAA Fuel Prices, national average (regular), $4.17/gal as of 2026-11-02. Same refresh rule.
- Utility EV rates: taken only from the utility's own tariff sheets or, where the sheet is not published as a single document, the utility's official rate page (marked draft until the sheet is read). Each plan carries the document title, URL, document date, effective date and the date we checked it. A plan whose check is more than 100 days old is labelled stale on the page.
What is not modelled
- Fixed charges (such as California's Base Services Charge), minimum bills, taxes and surcharges.
- Tiered or baseline pricing and baseline credits. Where a plan has one, the page says so and shows the pre-credit rate.
- Time-of-use allocation of the rest of the household's usage. The utility pages use one period's rate as a preset; they do not simulate a bill or recommend a plan.
- Community choice aggregator generation rates, solar exports, demand charges.
- Vehicle purchase, depreciation, insurance, maintenance and tires.
Verification
The calculation library has unit tests for boundary conditions and five reference scenarios computed independently in a second language with decimal arithmetic. Utility rates are re-read against their source documents in the first week of each quarter and whenever a reader or a rate-change notice points to a change.
Corrections
If a number on this site is wrong, tell us through the contact page with the source you are looking at. Corrections are made on the page, with the content-changed date updated; we do not silently rewrite history.