How to Calculate Electricity Costs (kWh)
Electricity is billed by the kilowatt-hour, so the cost of running anything comes down to three numbers: its power in watts, how long it runs, and your price per kWh. Here is the formula and how to read it.
The formula
Energy in kilowatt-hours = watts × hours ÷ 1000. Multiply that by your price per kWh for the cost. A 1500 W heater run for 3 hours uses 1500 × 3 ÷ 1000 = 4.5 kWh; at 0.30 per kWh that is 1.35.
What a kilowatt-hour is
One kWh is the energy a 1000-watt device uses in one hour. Your bill adds up all the kWh you use and multiplies by the tariff. Higher-wattage devices and longer run times both raise the cost, in direct proportion.
Reading the numbers realistically
The wattage printed on an appliance is usually its maximum draw, and many devices do not run flat out. A fridge cycles its compressor on and off, and a thermostat-controlled heater idles once the room is warm, so real use is often below the rated figure. Standby power from devices left plugged in also quietly adds up.
Find the biggest costs
The appliances that cost the most are the high-wattage ones that run for a long time: heating, cooling, water heaters, tumble dryers and ovens. Trimming their run time saves far more than switching off a low-watt gadget. The Electricity Cost Calculator shows the cost per day, month and year so you can spot them.
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Last updated: July 6, 2026