Methodology

1. Electricity rates by state

State-level residential rates are taken from the U.S. Energy Information Administration (EIA) Electric Power Monthly, Table 5.6.A. These are arithmetic averages across utilities operating in each state, in cents per kWh. We update the dataset monthly when the EIA publishes a new vintage. The current vintage used on this site is shown in the footer of every page.

2. Appliance running cost

For an appliance with nameplate watts, used hours/day for days/year at a rate of R cents/kWh, optionally with duty cycle d (fraction of on-time the appliance actually draws power):

cost_per_year ($) = watts × d × hours_per_day × days_per_year × R / 100 / 1000

Duty cycle accounts for cycling appliances such as refrigerators (~0.35) and hot tubs (~0.25). Resistive loads like space heaters have d = 1.0.

3. Solar production and savings

For a PV system of system_kW in a location with peak_sun_hours per day (the climatological average), with a combined performance ratio PR (inverter, temperature, and soiling losses):

annual_kWh = system_kW × peak_sun_hours × 365 × PR

Default PR = 0.78, a commonly cited industry value. Annual savings = annual_kWh × rate. Payback assumes a national-average installed cost of $3.00/W and the U.S. federal Investment Tax Credit (30% through 2032).

4. EV charging cost

For an EV that drives M miles/year and achieves E mi/kWh "at the wheels":

annual_kWh_from_grid = (M / E) / charging_efficiency

Default charging_efficiency = 0.90 for Level 2 home charging.

5. Limitations

State averages mask significant intra-state variation (different utilities, time-of-use plans, demand charges). Your bill may differ. The customization box on each calculator lets you override the rate with your actual cents/kWh.