Calculate energy before price
A kWh cost calculator turns an appliance’s electrical power, running time, and energy tariff into an estimated bill. Enter power in watts, the hours of use per day, your price for one kilowatt-hour, and the number of days to cover. Watts divided by 1,000 become kilowatts; kilowatts multiplied by hours become kilowatt-hours. Only then multiply by the price per kWh. A watt is a rate of energy use, whereas a kilowatt-hour is an amount of energy. Confusing the two is the main reason an otherwise simple cost calculation goes wrong. Read the tariff unit carefully before entering a price.
A 60 W worked example
Suppose a 60 W lamp runs for five hours each day and the variable energy price is 0.25 per kWh in your chosen currency. Daily energy is 60 × 5 ÷ 1,000 = 0.30 kWh. The daily energy charge is 0.30 × 0.25 = 0.075 currency units. Over 30 identical days, that is 9 kWh and 2.25 currency units. The price field does not assume a particular currency: enter a tariff and read the cost in that same currency. A month with a different number of days can be entered directly rather than forced into a 30-day assumption.
When the estimate is useful
This calculation helps compare two lamps, estimate a computer’s operating cost, or decide whether a device left on overnight matters on the bill. For a heater with a thermostat, nameplate power is the draw while heating; the average over a whole day can be much lower. A refrigerator or air conditioner also cycles instead of consuming its rated maximum continuously. If a smart plug or utility meter gives measured kWh, that observation is preferable to multiplying a nominal watt rating by hours. For a cost comparison, keep the tariff, period, and usage pattern consistent across alternatives.
Tariff and measurement limits
This page estimates the variable energy charge, not a complete electricity invoice. Fixed network fees, tiered rates, time-of-use prices, taxes, and discounts may change the final amount. Do not enter cents when you mean whole currency units: a tariff of 25 cents per kWh is 0.25 currency units per kWh. Hours per day cannot exceed 24, and days should reflect the chosen period. If the appliance has multiple power states, calculate each state separately and add their energy use rather than pretending it has one constant load. Round the displayed money for presentation only; keep the underlying kWh calculation before rounding.