The units of energy
Energy has more competing units than almost any other magnitude, because different fields measured it independently before anyone unified them. The joule is the SI unit: one newton acting over one metre. Electricity bills use kilowatt hours because a joule is far too small; food labels use calories or kilojoules; and the food Calorie with a capital C is really a kilocalorie, which is the single most common source of confusion here.
| Unit | Symbol | Equals |
|---|---|---|
| Joules | J | 1 J |
| Kilojoules | kJ | 1,000 J |
| Calories | cal | 4.184 J |
| Kilocalories | kcal | 4,184 J |
| Watt hours | Wh | 3,600 J |
| Kilowatt hours | kWh | 3,600,000 J |
How to convert by hand
Every energy conversion follows the same procedure: take the amount to the base unit, then from there to the target unit. In practice it is a single multiplication, because the two steps combine into one factor.
Convert to joules and back out. A kilowatt hour is 3.6 million joules, a kilocalorie is 4,184 joules, and a watt hour is 3,600 joules.
The equivalents people look up most
| Conversion | Result |
|---|---|
| A day of household electricity | 10 kWh · 36 MJ |
| A chocolate bar | 250 kcal · 1,046 kJ |
| A daily food intake | 2,000 kcal · 8,368 kJ |
| An electric car battery | 60 kWh · 216 MJ |
| 1 kWh | 3.6 MJ · 860 kcal |
Common mistakes
- Confusing the calorie with the Calorie. The food unit is a kilocalorie, a thousand times bigger than the scientific one.
- Mixing up kilowatts and kilowatt hours. One is a rate, the other an amount. A 2 kW heater for 30 minutes is 1 kWh.
- Comparing a battery's kWh with a fuel tank's litres directly. A litre of petrol holds about 9.7 kWh of chemical energy, but an engine wastes most of it as heat.
- Assuming energy and power are interchangeable. Power is energy per unit time, and the units are not convertible into each other.