Electricity Cost Calculator
Works out what an appliance costs to run over a day, a month and a year, from its wattage and your unit rate. Standby draw is included separately, because on always-plugged devices it is often the larger figure.
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The formula
cost = (watts ÷ 1000) × hours × rate
A kilowatt hour is a thousand watts for one hour. Divide the wattage by 1,000 to get kilowatts, multiply by the hours of use, then by the unit rate.
What to watch out for
Two things this gets right that a back-of-envelope estimate misses:
- Standby is counted for the hours the appliance is idle, not the hours it runs — 8,760 hours in a year minus the active ones. A 2 W standby draw left on all year is 17 kWh, which can exceed the running cost of something used briefly.
- Nameplate wattage is a maximum, not an average. A fridge rated 150 W cycles on and off and averages perhaps 40 W; a kettle rated 3,000 W really does draw 3,000 W, but only for three minutes. Use the average draw where you know it.
For anything with a duty cycle — fridges, freezers, heat pumps, air conditioners — an energy monitor gives a far better figure than the plate rating.
Frequently Asked Questions
How much does it cost to run a 1000 W appliance for an hour?
Exactly one kilowatt hour, so one unit rate. At 25p or 25 cents per kWh, that is 25p or 25 cents an hour.
Does standby power really matter?
It can. A device drawing 2 W continuously uses about 17.5 kWh a year. Across a dozen such devices that is 200 kWh — a real line on the bill for equipment nobody is using.
Why is my fridge cheaper than the label suggests?
Because it cycles. The plate rating is the compressor draw while running, and a fridge runs perhaps a third of the time. Use the annual kWh figure on the energy label instead.