METHODOLOGY

How WattBill estimates electricity cost.

Core formula

Daily energy cost = appliance watts ÷ 1,000 × equivalent operating hours per day × electricity price per kWh.

Monthly estimates use 30 days and annual estimates use 365 days. Currency results are displayed to two decimal places while calculations retain the underlying numeric precision.

State electricity rates

State residential averages are retrieved from the U.S. Energy Information Administration API, Electricity Retail Sales dataset, residential sector, monthly frequency. The newest available record for each state and the District of Columbia is used.

Data validation and storage

The update is rejected unless at least 50 valid state records contain numeric prices. Accepted data is stored in Cloudflare KV and edge cache. A scheduled Worker refreshes the dataset monthly; visitors receive the last validated copy when available.

Your bill is the better rate source

An EIA state average blends many utilities and customers. The price per kWh on a visitor's own bill is usually more accurate for personal cost estimates, particularly with time-of-use, tiered, or special EV rates.

Equivalent operating hours

Appliances such as refrigerators, dryers, heaters, and air conditioners may cycle or modulate. For those devices, hours should represent equivalent full-power time rather than every hour plugged in. Product EnergyGuide annual-kWh figures or measured energy are preferable when available.

What the estimate excludes

Fixed customer charges, taxes, demand charges, minimum bills, fuel adjustments, tier thresholds, charging-network fees, and appliance performance changes are not included unless explicitly entered as part of the rate.

WattBill USA provides educational estimates, not engineering, financial, or utility-billing advice.