How We Calculate Costs

Every number on Billowatt is computed from two inputs: current state-level electricity rates and realistic appliance power data. This page documents both, plus the formulas connecting them, so you can judge — and adjust — every estimate we publish.

Electricity rates

We use the average residential retail electricity price per state published by the U.S. Energy Information Administration (EIA), currently reflecting April 2026 data. Rates are refreshed monthly as the EIA publishes new figures. The current national average in our dataset is 18.8¢/kWh.

A state average is exactly that — your utility, plan, and tier may differ, and your effective rate (bill total ÷ kWh, including delivery charges and taxes) is typically somewhat higher than nominal supply rates. For maximum accuracy, compute your effective rate from a recent bill and use it in our calculator, which accepts any rate via the state selector's underlying math.

Appliance power data

Our database covers 157 appliances. For each we publish a typical wattage and a realistic range, drawn from U.S. Department of Energy and ENERGY STAR published figures, manufacturer specifications, and the physics of the device class (for example, resistance heaters converting electricity to heat 1:1). Two usage models are used:

• Continuous devices (heaters, TVs, fridges): typical watts × typical hours per day. For cycling devices like refrigerators, the wattage shown is an effective average that already accounts for compressor duty cycle.
• Session devices (dishwashers, dryers, EV charging): energy per use (kWh) × uses per week.

"Typical usage" assumptions are stated on every appliance page and are deliberately ordinary — an 8-hour winter day for a space heater, five dryer loads a week for a family. Your habits will differ; that's why every page embeds a calculator with adjustable inputs.

The formulas

• kWh/day = watts ÷ 1,000 × hours/day (continuous) or kWh/use × uses/week ÷ 7 (session)
• Cost/day = kWh/day × rate
• Cost/month = cost/day × 30.4 (average month length)
• Cost/year = cost/day × 365

Where standby power is meaningful, kWh/day adds standby watts × non-active hours. Monthly figures for seasonal appliances (AC, holiday lights) describe an in-season month, noted on those pages.

What our numbers are — and aren't

These are planning estimates, not measurements of your specific devices, and not billing predictions. They're built to be honest at the "which of these is big?" level that drives good decisions. For your own exact figures, a plug-in energy meter is cheap and definitive — we explain the method in How to Measure What Any Appliance Really Uses.

Corrections

If you believe a figure is wrong, we want to know. Reach us via the contact page — corrections are reviewed against primary sources and published promptly.