Methodology
EV charging cost — methodology
The exact formulas, assumptions, default values and limitations behind the EV charging cost calculator.
Last updated
This documents the EV Charging Cost Calculator.
What this calculator does
It works out what it costs to charge an electric car: per full charge, per top-up session over a state-of-charge window you choose, per mile, and per month and year. It also compares charging at home against a representative public rate. Costs are billed on energy drawn from the wall — after charging losses — because that is what your meter records and what you pay for.
The formulas
Energy drawn from the wall (battery energy ÷ charging efficiency)
energy_full_wall = battery_kwh / charge_efficiency
energy_session_wall = battery_kwh * (target_soc - start_soc) / 100 / charge_efficiency
Cost per charge and per session (at the home rate)
cost_per_full_charge = energy_full_wall * home_price_per_kwh
cost_per_session = energy_session_wall * home_price_per_kwh
Cost per mile (price ÷ effective miles per kWh from the wall)
cost_per_mile = home_price_per_kwh / (miles_per_kwh * charge_efficiency)
Monthly and annual cost
annual_energy_wall = annual_miles / miles_per_kwh / charge_efficiency
annual_cost_home = annual_energy_wall * home_price_per_kwh
annual_cost_public = annual_energy_wall * public_price_per_kwh
monthly_cost_home = annual_cost_home / 12
We guard every division: a zero charging efficiency or zero efficiency (mi/kWh) returns “no result” rather than infinity, so a bad input never shows a misleading number.
Default values
Every default below is editable in the calculator and shown with its source in the calculator’s assumptions table. The home electricity price is the most time-sensitive — always replace it with your own rate, and use your off-peak EV rate if you have one.
- Home electricity price: $0.17/kWh (US residential average).
- EV efficiency: 3.5 mi/kWh.
- Charging efficiency: 88% (wall-to-battery, AC charging).
- Public / DC fast price: $0.40/kWh — a representative figure you supply.
- Battery, charge window and mileage: your car and usage.
Limitations — read these
- Charging losses are modelled, not measured. We use a single 88% charging efficiency for AC charging. DC fast charging, cold weather and a near-full battery (where charging tapers) all change real losses. - Public price is one representative rate. Real public and DC fast charging varies widely by network, location and time, and is sometimes billed per minute rather than per kWh. Treat the public figure as illustrative. - No demand charges or free charging. We ignore network session/idle fees, demand charges, and free workplace or destination charging — all of which can move your real cost up or down.
How we keep it honest
The calculation logic lives in a small, pure function that is unit-tested against normal, edge and invalid inputs (for example, a zero charging efficiency returns “no result” rather than infinity). If you spot an error, tell us and we’ll fix it.
Sources
Every default in this calculator traces to one of these.
- Average price of electricity to ultimate customers (residential)U.S. Energy Information Administration (EIA) · retrievedResidential prices vary by state and change monthly. Treat the default as representative and edit to your own rate.
- Fuel economy ratings (mpg) and EV efficiency (mi/kWh)U.S. DOE / EPA — fueleconomy.gov · retrieved
- Electricity (EV) — vehicle efficiency and charging lossesU.S. DOE Alternative Fuels Data Center (AFDC) · retrievedAC charging losses commonly add ~10–15% to energy drawn from the wall (charge efficiency ~0.85–0.90).
By EnergyTally Team · Editorial & analysis team
- Published
- Updated
- Review
- EnergyTally Team,