Electrical Energy: E = P × t
Energy is power sustained over time. A 2 kW kettle running for 5 minutes uses the same energy as a 10 W LED bulb running for 100 minutes. The kilowatt-hour (kWh) is the billing unit — 1 kWh = 1000 watts for 1 hour.
P — Power in watts (W). The rate of energy use. 1 kW = 1000 W.
t — Time in seconds (s). For kWh: use hours directly.
£ Cost — kWh × electricity rate. UK average ~£0.28/kWh (2025).
Electrical Energy Calculator
Power is the rate of energy use. Energy is power sustained over time. E = P × t. Enter any two — the calculator solves for the third, converts to kilowatt-hours (the billing unit on your electricity meter), and optionally calculates cost. A 2 kW kettle uses in 5 minutes what a 10 W LED uses in over 16 hours.
The Three Formulas
P = E / t — find power from energy and time
t = E / P — find time from energy and power
Leave one field empty. The calculator fills it. Add an electricity rate (£/kWh) to see the cost.
Unit Conversions
1 Wh = 3,600 J
1 W = 1 J/s
kWh is the unit on electricity bills. To get kWh directly: power in kW × time in hours = kWh.
Kettle (2 kW, 5 Minutes)
Or directly: 2 kW × (5/60) h = 0.167 kWh
Cost at £0.28/kWh: 0.167 × 0.28 = £0.047 ≈ 5p per boil
5p to boil a kettle. Do it 5 times a day and it is 25p/day, about £91/year. Small per-use costs add up. To find the current the kettle draws from the mains, use the Ohm’s Law Calculator — at 230 V, a 2 kW kettle draws about 8.7 A.
LED Bulb (10 W, 8 Hours)
Cost: 0.08 × 0.28 = £0.022 ≈ 2p per evening
2p for an evening’s lighting. An old 60 W incandescent doing the same job: 0.48 kWh = 13p. Replacing one bulb saves ~£40/year at 8 hours/day. For the resistor needed to drive an LED from a DC supply, see the LED Resistor Calculator.
Electric Oven (2.5 kW, 1 Hour)
Cost: 2.5 × 0.28 = £0.70
70p per cooking session. The oven cycles on and off to maintain temperature, so actual consumption is typically 60–80% of the rated power — roughly 1.5–2 kWh for an hour of use. The calculator gives the maximum; real usage is usually lower.
Space Heater (2 kW, 10 Hours/Day, 30 Days)
Cost: 600 × 0.28 = £168/month
Electric heating is expensive. 600 kWh/month from a single heater can double or triple a household electricity bill. This is the calculation that motivates heat pump adoption — a heat pump delivers the same heat for roughly one-third the electricity. To understand how the heater’s power relates to voltage and current, the Power Calculator shows P = V × I.
Reverse Solve — Find Time
“How Long Does 1 kWh Last at 3 kW?”
A 3 kW device burns through 1 kWh (one billing unit) in just 20 minutes. Useful for understanding how quickly high-power appliances run up the meter.
“How Long Can a 100 Wh Battery Run a 15 W Device?”
This is the battery runtime question. A laptop with a 100 Wh battery drawing 15 W average runs for about 6.5 hours. At 30 W (heavy use): 3.3 hours. For dedicated battery runtime calculations, see the Battery Runtime Calculator.
“How Long to Use £5 of Electricity at 2 kW?”
t = 17.86 / 2 = 8.93 hours
About 9 hours of 2 kW heating for £5. Work backwards from your budget to find how long you can run a device.
Reverse Solve — Find Power
“My Bill Shows 350 kWh Over 30 Days — What is My Average Power?”
486 W average, around the clock. This includes everything — fridge, lighting, standby loads, cooking, heating. If it seems high, something is running constantly that should not be. The calculator helps you work backwards from the bill to find the culprit.
Common Appliance Energy Use
Kettle boil: ~0.17 kWh (5p)
Washing machine cycle: ~1.5 kWh (42p)
Dishwasher cycle: ~1.2 kWh (34p)
Tumble dryer cycle: ~2.5 kWh (70p)
Per hour:
LED bulb: 0.01 kWh (0.3p)
TV (50"): 0.08 kWh (2p)
Desktop PC: 0.15 kWh (4p)
Electric oven: ~1.5 kWh (42p, cycling)
Space heater: 2 kWh (56p)
Power vs Energy — The Key Distinction
Power (watts) is the rate — how fast energy is being used right now. A 3 kW kettle uses energy fast. A 10 W LED uses it slowly. Both are measured by the same meter.
Energy (kWh) is the total — power × time. The kettle uses 0.17 kWh per boil. The LED uses 0.08 kWh per evening. Over a year the LED’s total energy may exceed the kettle’s, because it runs far more hours despite its lower power.
Your electricity bill charges for energy (kWh), not power (kW). A 10 kW device running for 6 minutes costs the same as a 10 W device running for 100 hours — both consume 1 kWh.
Frequently Asked Questions
Last updated: March 2026