Personal finance
Electricity usage and cost at home
List the appliances with their power and hours of use per day, enter the price per kWh from your bill: get the daily, monthly and yearly usage and cost, and see which appliances weigh most.
From watts to the bill
The power in watts says how much energy an appliance draws at each moment; multiplied by the hours of use it gives watt-hours, and dividing by 1000 gives kWh, the unit on the bill. A 2000 W oven on for half an hour uses 1 kWh: at 0.30 per kWh that is 30 cents.
Hours matter far more than power. A 2 kW heater on 4 hours a day for 120 days uses 960 kWh, about 288 at 0.30 per kWh; a 10 W router on all the time uses 88 kWh a year, under 27. Standby counts too: 5 W left on around the clock is almost 44 kWh a year.
Fridges and freezers are a special case: the compressor switches on and off, so the rated power must not be multiplied by 24 hours. Start from the yearly usage on the energy label and divide it by 8.76 to get the average power in watts.
Common mistakes
- Using the energy rate alone: standing charges, network costs and taxes can nearly double the real cost per kWh.
- Multiplying the rated power of a fridge or air conditioner by all the hours it is on: compressors run in cycles.
- Mixing up kW and kWh: the first is a power, the second an energy, power times time.
Frequently asked questions
How do I find my price per kWh?
Divide the total of the bill by the kWh used in the same period: that gives the effective price with every charge included, which is the one to use here.
Where do I read an appliance's power?
On the rating plate, usually on the back or underneath, or in the manual. For more precision, plug-in meters record the real kWh.
How much does a home use on average?
It varies widely by country and heating: a European household typically uses 2000 to 4000 kWh a year, far more with electric heating, a heat pump or an electric car.
How this calculation works
Daily energy of an appliance (kWh) = power (W) × hours per day × quantity / 1000. Yearly usage = daily × 365, monthly = yearly / 12. Cost = kWh × price per kWh. Average power drawn = kWh per day × 1000 / 24. Each appliance's share is its usage divided by the total.
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