Kilowatt-hours to joules
Convert kilowatt-hours to joules. One kWh is exactly 3,600,000 J, with appliance, battery and electricity bill tables.
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The formula
joules = kWh × 3,600,000The factor is exact and easy to derive: a watt is one joule per second, a kilowatt is 1000 joules per second, and an hour is 3600 seconds. So a kilowatt-hour is 1000 × 3600 = 3.6 million joules, or 3.6 megajoules.
Why the kilowatt-hour exists at all
It looks like an awkward compound unit, and it is — but it is the natural one for electricity because a meter measures power continuously and bills for the integral.
The name states the definition: a kilowatt, sustained for an hour.
| Appliance | Power | Time | Energy |
|---|---|---|---|
| Kettle | 2.5 kW | 4 min | 0.17 kWh |
| Microwave | 1 kW | 10 min | 0.17 kWh |
| Washing machine | 0.5 kW | 1 h | 0.5 kWh |
| Air conditioner | 1.1 kW | 8 h | 8.8 kWh |
| Fridge | 0.15 kW | 24 h | 3.6 kWh |
| LED bulb | 0.01 kW | 5 h | 0.05 kWh |
An air conditioner run overnight uses as much energy as a kettle boiled fifty times.
Quick reference
| kWh | Joules | Megajoules | kcal |
|---|---|---|---|
| 0.1 kWh | 360,000 | 0.36 MJ | 86 |
| 1 kWh | 3,600,000 | 3.6 MJ | 860 |
| 5 kWh | 18,000,000 | 18 MJ | 4302 |
| 10 kWh | 36,000,000 | 36 MJ | 8604 |
| 60 kWh | 216,000,000 | 216 MJ | 51,625 |
| 100 kWh | 360,000,000 | 360 MJ | 86,042 |
Batteries
Battery capacity is quoted in kWh for vehicles and storage, and in mAh for phones and laptops — which is not an energy at all until it is multiplied by a voltage:
| Capacity | Joules | |
|---|---|---|
| Phone, 4000 mAh at 3.85 V | 0.0154 kWh | 55,440 |
| Laptop, 60 Wh | 0.06 kWh | 216,000 |
| Home storage battery | 10 kWh | 36,000,000 |
| Small EV | 40 kWh | 144,000,000 |
| Large EV | 100 kWh | 360,000,000 |
A phone battery holds about 55 kJ, which is roughly the food energy of three grams of sugar.
Against food and fuel
| kWh | MJ | kcal | |
|---|---|---|---|
| Daily food intake | 2.32 | 8.37 | 2000 |
| Litre of petrol | 9.5 | 34.2 | 8170 |
| Litre of diesel | 10.7 | 38.6 | 9220 |
| Cubic metre of natural gas | 10.6 | 38.3 | 9150 |
A litre of petrol carries about four times a person’s daily food energy — though a car converts only a fraction of it into motion, and a body is considerably more efficient than that.
Estimating in your head
Multiply by 3.6 and add six zeros. Or work in megajoules: kWh × 3.6 is MJ directly, and MJ is a friendlier size for anything that is not an electricity bill.