What Is a Kilowatt-Hour? Reading Your Electricity Bill
A kilowatt is a rate. A kilowatt-hour is an amount. Confusing the two is why electricity bills feel mysterious.
30 March 2026 · 6 min read · Energy
Your electricity meter counts kilowatt-hours. Your appliances are labelled in watts. Understanding the link makes a bill readable.
Power versus energy
Power is the rate of energy use, measured in watts. Energy is power multiplied by time, measured in kilowatt-hours.
kWh = (watts ÷ 1000) × hours
A 2,000 W kettle running for 6 minutes (0.1 h) uses 2 × 0.1 = 0.2 kWh.
In SI terms, 1 kWh = 3.6 megajoules. The kWh to megajoules converter handles it.
What things actually cost
At £0.25 / $0.25 per kWh:
| Appliance | Power | Typical use | Cost |
|---|---|---|---|
| LED bulb | 9 W | 5 h/day | £0.011/day |
| Laptop | 60 W | 8 h/day | £0.12/day |
| Fridge-freezer | 150 W (cycling) | ~1.2 kWh/day | £0.30/day |
| Electric oven | 2,100 W | 1 h | £0.53 |
| Tumble dryer | 2,500 W | 1.5 h | £0.94 |
| EV charge | 7,000 W | 8 h (56 kWh) | £14.00 |
The pattern is clear: things that make heat dominate a bill. Standby electronics, the traditional villain, are now typically 1–3% of household use.
Reading the meter
A meter reading is cumulative. Subtract last month's reading from this month's to get the kWh consumed. If your bill says "estimated", the supplier guessed — submit a real reading and the next bill corrects itself.
Solar and battery sizing
- Panels are rated in kilowatts peak (kWp) — output under standard test conditions.
- Batteries are rated in kilowatt-hours — how much they store.
A 4 kWp array in the UK generates roughly 3,400 kWh per year; the same array in Arizona generates about 6,400 kWh. Latitude, shading and orientation matter more than the panel brand.
Units per hour, and other nonsense
Watch for meaningless phrases on marketing material. "Kilowatts per hour" is not a unit of anything useful — kilowatts already contain time. If a document says kW/h, it almost always means kWh, and it is a sign the document was not checked.
Related conversions
This article is for general information. Please verify figures against the relevant standard before using them in critical work. About accuracy