Methodology

Every calculator runs in your browser. Nothing you type is sent to us. Each uses a short formula, listed here.

Appliance electricity cost

kWh per day = watts / 1000 x hours per day. Cost = kWh x price per kWh. A month is 365 / 12 days.

EV vs gas car cost

EV cost per mile = (kWh per 100 miles / 100) x price per kWh. Gas cost per mile = price per gallon / MPG. Yearly cost = cost per mile x miles per year.

Solar payback

First-year kWh = kW x peak sun hours x 365 x system efficiency. In year n, output = first-year kWh x (1 - degradation)^(n-1) and price = rate x (1 + increase)^(n-1). Payback is when cumulative savings reach the net cost (system cost after incentives), interpolated within that year. We stop at 30 years.

Default values

Defaults such as 17 cents per kWh, $3.50 per gallon and $3.00 per watt are round example figures, not live prices. Replace them with your own numbers for a useful result. We review defaults periodically.

Testing

Each calculator has automated tests that compare its output with hand-calculated answers, and checks that bad inputs are rejected.

Limits

Results are estimates. They do not include taxes, fees, financing, or rules specific to your utility. See the disclaimer.