Electricity Cost Calculator
Find out what any appliance costs to run from its power rating, how long it is on each day and your electricity tariff. Enter the watts, the daily hours and your pence per kWh to see the cost per day, week, month and year, plus the kWh it uses.
Inputs
Results
kWh per day = watts times hours divided by 1,000. Cost = kWh times your unit rate. The monthly figure is the yearly cost divided by 12, and the daily standing charge is not included.
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How the appliance running cost is worked out
This calculator turns an appliance's power rating into money. Power in watts times hours of use gives watt-hours; dividing by 1,000 gives kilowatt-hours, the unit your electricity bill charges by. Multiplying by your tariff's unit rate in pence per kWh produces the running cost, which the calculator scales to a day, a week, a month and a year.
The yearly figure is the eye-opener for always-on devices. Something drawing a steady 100 watts around the clock uses about 876 kWh a year, which at a typical unit rate is a three-figure sum for a single device. Conversely, high-power appliances used briefly, like a 3 kW kettle for a few minutes, cost less than intuition suggests.
The calculation deliberately excludes the standing charge, the fixed daily amount you pay for being connected. That makes the result the marginal cost of the appliance: what you would actually save by switching it off.
How to use the electricity cost calculator
- Find the appliance's power in watts from its rating plate, manual or a plug-in energy monitor.
- Enter how many hours a day it actually runs. For cycling appliances like fridges, use an estimated average.
- Enter your unit rate in pence per kWh from your bill or tariff.
- Read the cost per day, week, month and year, plus the kWh used per day.
Worked examples
A 100 watt device on 6 hours a day at 26p per kWh
Inputs: Power 100 watts, 6 hours per day, 26 pence per kWh.
Result: 0.6 kWh per day costing about 16p, which is roughly 1.09 a week, 4.75 a month and 56.94 a year.
A 2,000 watt heater on 3 hours a day
Inputs: Power 2000 watts, 3 hours per day, 26 pence per kWh.
Result: 6 kWh per day costing about 1.56, which compounds to roughly 47.45 a month over a cold spell.
Limitations and common mistakes
Edge cases and limitations
- Many appliances do not draw their nameplate rating constantly. Fridges, washing machines and ovens cycle, so their average draw is well below the maximum; a plug-in meter gives the true figure.
- The unit rate is assumed flat. Time-of-use tariffs with cheap overnight windows need the rate for the hours the appliance actually runs.
- The standing charge is excluded on purpose, so this is the marginal cost of the appliance rather than a share of your whole bill.
- Monthly cost is the annual figure divided by 12, so it will not match a calendar month exactly.
Common mistakes
- Reading the maximum rating as the constant draw. A 900 watt microwave only draws that while heating, not while its clock is on standby.
- Entering the rate in pounds instead of pence. A unit rate of 0.26 instead of 26 makes everything look one hundred times cheaper.
- Ignoring standby power for devices that are never truly off; a few watts around the clock adds up over a year.
Frequently asked questions
How do I work out how much an appliance costs to run?
Multiply the appliance's power in kilowatts by the hours it runs to get kWh, then multiply by your unit rate. A 2000 watt heater on for 3 hours uses 6 kWh; at 26p per kWh that is about 1.56 per day. The calculator does this and scales it to weekly, monthly and yearly figures.
Where do I find an appliance's wattage?
Check the rating plate or sticker on the appliance itself, the manual, or the manufacturer's website. Note that many appliances do not draw their maximum rating constantly: a fridge cycles on and off and a washing machine only heats water for part of the cycle, so an average figure or a plug-in energy monitor gives a more realistic number.
What unit rate should I enter for electricity?
Use the pence per kWh unit rate from your own bill or tariff, as rates vary by supplier, region and tariff type. The calculator's result excludes the daily standing charge, which you pay regardless of usage, so it shows the marginal cost of running the appliance.
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