The electricity-cost formula
We convert watts to kilowatts, multiply by hours of use, and then multiply by the electricity price expressed in dollars per kilowatt-hour.
(watts ÷ 1,000) × hours used × (cents per kWh ÷ 100) = estimated cost
Daily estimates use the hours you enter. Monthly estimates use 30 days, and yearly estimates use 365 days. These are arithmetic estimates, not modeled utility bills.
Electricity prices
State and national residential prices come from the U.S. Energy Information Administration. Pages display the source period next to the rate. A utility plan, time-of-use tariff, tax, fee, or fuel adjustment can make your effective price different, so the calculator always lets you replace the default.
Appliance wattage and runtime
Default wattage and runtime are representative starting points, not promises about a specific product. Actual draw can vary with capacity, efficiency, duty cycle, thermostat behavior, age, and operating mode. Start with the nameplate, energy label, or manufacturer input specification. For a compatible plug-in appliance, you can also use a power meter rated for its voltage, current, and load type. For hardwired or high-power equipment, use manufacturer specifications, circuit-level monitoring, or a qualified electrician instead.
How updates are checked
The update process keeps source numbers separate from the writing around them. Code retrieves public data, checks units and ranges, rejects missing or duplicate locations, and recalculates every displayed cost. Drafting tools may help prepare short explanations, but they cannot supply the source numbers and their output must pass the same content schema before publication.
If any check fails, the current published data stays in place for review.
Rounding
Costs are calculated at full precision and rounded only for display. Currency normally appears to the nearest cent; energy use may appear to one decimal place. Adding rounded daily figures will occasionally differ by a cent from the displayed monthly figure.
What the estimate leaves out
- Utility fixed charges, taxes, demand charges, and tiered pricing.
- Seasonal efficiency changes and equipment cycling unless a page states otherwise.
- Solar production, battery storage, rebates, or net-metering credits.
- Maintenance, financing, installation, and equipment purchase costs.