Electricity Cost Calculator
Wattage times hours times tariff. The surprise is usually which appliances turn out to matter and which do not.
The calculation
A kilowatt-hour is one kilowatt drawn for one hour. A 2,000 W heater running for two hours uses 4 kWh; at 27p per kWh that is £1.08.
The rating on the label is maximum draw, not average. A fridge rated 150 W does not draw 150 W continuously — its compressor cycles, so actual consumption might be a third of that. The same applies to washing machines, dishwashers and ovens with thermostats. For those, the energy label's annual kWh figure is far more reliable than the wattage.
What actually costs money
Typical annual costs at 27p/kWh, showing how much the answer depends on heat:
| Appliance | Typical use | Annual cost |
|---|---|---|
| Electric heater, 2 kW | 4 h/day, 5 winter months | ~£325 |
| Tumble dryer | 3 cycles a week | ~£120 |
| Fridge-freezer | Continuous | ~£75 |
| Electric shower, 9 kW | 10 min/day | ~£145 |
| Kettle | 5 boils a day | ~£45 |
| Washing machine | 4 cycles a week | ~£35 |
| Gaming PC, 400 W | 3 h/day | ~£120 |
| LED bulb, 9 W | 5 h/day | ~£4.40 |
| Phone charger | Daily | ~£0.50 |
The pattern is consistent: anything that makes heat or moves heat dominates the bill. Electronics and lighting, which people worry about most, are usually trivial by comparison. Switching every bulb in a house to LED saves real money; unplugging a phone charger saves almost nothing.
Standby and the vampire load
Standby consumption has fallen sharply — EU rules cap most devices at 0.5 W — but it adds up across a house. A typical home has 10 to 20 devices drawing standby power continuously, totalling perhaps 30–60 W, or £70–£140 a year.
The worst offenders are not the obvious ones. Set-top boxes and games consoles in “instant on” mode can draw 10–20 W around the clock; a television on standby draws well under one. If you want to reduce standby load, find the few devices drawing double-digit watts rather than unplugging everything.
Check your actual tariff. Unit rates vary enormously by country, supplier and time of day, and many tariffs add a daily standing charge regardless of use. Time-of-use tariffs can make running the dishwasher overnight several times cheaper than at 6pm.
Frequently asked questions
Is it cheaper to leave the heating on low all day?
Generally no. A house loses heat in proportion to the temperature difference with outside, so keeping it warm all day loses more heat in total than letting it cool and reheating. The exception is a very well-insulated home with a heat pump, which runs most efficiently at a steady low output.
How much does an electric shower cost per use?
A 9 kW shower uses 1.5 kWh in ten minutes — about 40p at 27p/kWh. A four-person household showering daily spends roughly £590 a year on hot water from that one appliance.
Do smart meters reduce bills?
Not by themselves — they measure rather than save. Their value is visibility: seeing consumption in real time tends to change behaviour, and they enable time-of-use tariffs where shifting heavy loads to cheap periods produces genuine savings.