How to Calculate Electricity Cost from Watts and Hours

Watts measure power; kilowatt-hours measure energy

Watts describe the rate at which a device uses energy. A kilowatt is 1,000 watts. A kilowatt-hour is the energy used by a constant 1-kW load over one hour, or an equivalent combination of power and time.

A 100-W device operating for ten hours uses the same 1 kWh as a 1,000-W device operating for one hour. Comparing wattage alone therefore cannot establish which appliance costs more over a month; operating time also matters.

Kilowatts = watts ÷ 1,000
Energy in kWh = watts × hours ÷ 1,000
Estimated energy cost = kWh × price per kWh

Use a rate quoted per kilowatt-hour, not per watt or per day. If the tariff says 20 cents per kWh, enter 0.20 in dollars per kWh. Entering 20 while interpreting the output as dollars would overstate the estimate by a factor of 100.

Choose wattage and operating hours that fit the appliance

A rated or nameplate wattage is not necessarily the device’s average draw throughout every hour it is switched on. Heaters, refrigerators and other controlled loads can cycle; electronics can use different power in different modes. Starting surges are also different from continuous consumption.

For a constant load, use its operating watts and hours directly. For a variable load, measured energy over a representative period can be more useful than multiplying a maximum rating by every connected hour. A compatible plug-in energy meter may provide watts and accumulated kWh; check the device’s instructions and rating rather than assuming any meter suits any appliance.

If you already measured total kWh, multiply that energy by the rate without multiplying by hours again. For example, 3 kWh measured over a day is already a daily energy total. Treating it as 3 kW and multiplying by 24 would describe a different quantity.

Worked example across a day, month and year

Assume a device draws a constant 1,000 W for two hours every day, has zero standby draw for this first example, and electricity costs an illustrative $0.20/kWh.

Period	Energy calculation	Energy used	Energy cost
One day	1,000 × 2 ÷ 1,000	2 kWh	$0.40
A 30-day month	2 × 30	60 kWh	$12.00
A 365-day year	2 × 365	730 kWh	$146.00

Twelve 30-day months account for only 360 days, so $12 × 12 is not the same as the $146 annual estimate. The Electricity Cost Calculator uses your entered operating days for its monthly estimate and a daily pattern repeated over 365 days for the annual estimate. For seasonal equipment, that annual pattern needs adjustment rather than being read as a forecast of your actual year.

The rate is an arithmetic assumption, not a claimed local tariff. Select your own currency label and price consistently; the calculator does not fetch rates or convert currencies.

Include standby power without overlapping active time

Standby means power drawn outside the entered active hours. If the same device draws 2 W during the remaining 22 hours each day, standby energy is 2 × 22 ÷ 1,000 = 0.044 kWh daily.

Daily kWh = (active watts × active hours
             + standby watts × (24 − active hours)) ÷ 1,000

The combined daily use is 2.044 kWh. At $0.20/kWh, that costs $0.4088 per day, $12.264 for 30 days and $149.212 for 365 days. Displayed to cents, those amounts are $0.41, $12.26 and $149.21. Round at the final display stage; multiplying the already-rounded $0.41 by 365 produces a different estimate.

Do not add standby across all 24 hours when active consumption already covers part of the day. For several identical appliances, multiply energy by quantity once. For devices with different schedules or wattages, use separate calculator rows.

Compare the estimate with the electricity bill

A flat per-kWh rate estimates an energy charge, not necessarily the whole bill. Fixed service charges do not increase in proportion to this appliance’s runtime. Delivery charges, taxes, tiered rates, demand charges and other tariff items can require separate treatment depending on the bill.

Time-of-use pricing needs the energy used in each rate period. Calculate each period’s kWh × its own rate, then add the costs. A simple flat-rate calculation cannot reproduce that tariff unless you supply an appropriate weighted average and accept its limitations.

The calculator supports an optional fixed monthly charge, adding it once across the appliance list rather than once per device. Its annual fixed-charge estimate uses twelve monthly charges. Verify which charges your entered energy rate already includes before adding them separately.

For a household plan, distinguish estimated appliance energy from the amount you actually pay on a bill, and compare representative periods. The monthly savings guide can help allocate a budget, but a lower calculated energy total does not guarantee an identical reduction in a bill with fixed or variable charges.