Electricity Cost Calculator

Watts measure power (rate of energy use). Kilowatt-hours measure energy (total consumption over time). To convert: multiply the device wattage by the number of hours it runs, then divide by 1,000. A 500 W gaming PC running for 4 hours uses 500 * 4 / 1,000 = 2 kWh. Multiply that by your tariff to get the cost for that session.

Several things reduce real-world consumption below the calculator estimate. Many devices do not run at rated wattage continuously. A gaming PC draws 500 W under full load but only 80-120 W at desktop idle. Devices in standby still draw power (1-5 W each), but far less than active use. If multiple people share a meter, the bill reflects the household total, not one device. The calculator assumes constant draw at the wattage you enter, so it gives a useful upper bound rather than a precise figure.

The most common cause is a tariff mismatch: the per-kWh rate you entered does not match what your supplier actually charges. Bills often carry a network delivery charge, taxes and VAT on top of the energy rate, and those can add 30 to 60 percent to the headline unit price. To get an accurate estimate, divide your last bill total (all in) by the kWh shown on that same bill. Use that blended figure in the tariff field rather than the energy-only rate your supplier advertises. Peak and off-peak contracts add another layer: a heat pump or EV charger running overnight may be on a cheaper rate than daytime devices, so the single-rate figure overstates those costs.

For most devices, energy cost scales roughly linearly with power draw; half the wattage means half the cost per hour. However, some devices are less efficient at very low loads. Variable-speed motors (heat pumps, inverter air conditioners) are most efficient in a mid-range band. Switching power supplies (laptop chargers, LED drivers) can be slightly less efficient at 5-10% load versus 50-80% load. For practical home energy calculations, treating cost as linear with wattage is accurate enough.