How to calculate electricity cost
Every appliance has a power rating in watts (W) or kilowatts (kW). Electricity suppliers charge for kilowatt-hours (kWh). A 2,000 W heater running at full power for one hour uses 2 kWh.
Power (kW) × time (hours) × unit rate = running cost
Common reasons to use Wattwise
Before buying a new appliance — compare a tumble dryer, air fryer or portable heater against what you already run. EV charging — see the monthly cost of a 7 kW wallbox versus a 2.3 kW granny charger, and model moving charge time into a cheaper night window. Whole-house planning — add every item you use regularly and adjust hours to see where the bill adds up. Economy 7 households — turn on advanced rates to split day and night use per appliance.
Typical appliance wattages
Ratings differ by model, and many appliances do not draw their maximum power continuously. Thermostats cycle on and off, while washing machines and dishwashers usually reach peak power when heating water. Use the rating plate for the most precise result.
Day and night electricity tariffs
Economy 7 and other time-of-use tariffs provide cheaper electricity during an off-peak window. Exact hours depend on your supplier, meter, region and daylight-saving arrangements. Wattwise starts at 11:00 pm to 7:00 am, but the advanced settings let you enter the times and rates shown on your tariff.
EV charging costs
A 7 kW wallbox charges faster than a portable 10 A “granny charger”, which typically draws around 2.3 kW. Total cost depends on energy delivered rather than charging speed, though real charging also has conversion losses. Moving charging into your cheap-rate window can reduce the cost considerably.
Frequently asked questions
How accurate is the calculator?
It is accurate for the figures entered. Actual consumption varies when appliances cycle, use variable power or enter standby. A plug-in energy monitor gives the best measured figure.
Does a 2,000 W heater always use 2 kWh an hour?
At full power, yes. A thermostat may switch the element off after reaching temperature and reduce average consumption.
Is a night tariff always cheaper?
Not necessarily. Some dual-rate tariffs charge more during the day, so savings depend on how much usage you can move off-peak.
Are EV charging losses included?
No. Actual battery energy gained can be lower because of vehicle, cable, temperature and conversion losses.