Energy units appear everywhere: electricity bills (kWh), nutrition (kcal), thermodynamics (joules), heating (BTU). Understanding the conversions between these units is useful for comparing electrical appliances, calculating energy savings or solving physics problems.
The joule (J) is the SI unit of energy. The kilowatt-hour (kWh) is the practical unit for electricity: 1 kWh = the energy supplied by a power of 1 kW for 1 hour = 3,600,000 joules. The calorie (cal) and kilocalorie (kcal) measure food energy: 1 kcal = 4,186 joules. The BTU (British Thermal Unit) is used in Anglo-Saxon heating/air conditioning systems.
📐 Formula
1 kWh = 3,600,000 J = 3,600 kJ | 1 kcal = 4,186 J | 1 BTU = 1,055 J
📊 Reference table
| Unit | Equivalent in joules | Equivalent in kWh | Domain |
|---|---|---|---|
| 1 joule (J) | 1 J | 2.78 × 10⁻⁷ kWh | Physics, SI |
| 1 kilojoule (kJ) | 1,000 J | 2.78 × 10⁻⁴ kWh | Nutrition, chemistry |
| 1 calorie (cal) | 4.186 J | — | Physics (historical) |
| 1 kilocalorie (kcal) | 4,186 J | 1.16 × 10⁻³ kWh | Food/nutrition |
| 1 kWh | 3,600,000 J | 1 kWh | Electricity |
| 1 BTU | 1,055 J | 2.93 × 10⁻⁴ kWh | Heating (US/UK) |
| 1 MWh | 3.6 × 10⁹ J | 1,000 kWh | Industry, grid |
💡 Practical examples
Example 1: electrical appliance consumption
An oven at 2,000 W (2 kW) used for 30 min: energy = 2 kW × 0.5 h = 1 kWh = 3,600,000 J. At $0.15/kWh, cost = $0.15.
Example 2: food calories in kWh
A 700 kcal meal = 700 × 4,186 = 2,930,200 J = 0.814 kWh. The human body consumes about 2,000 kcal/day ≈ 2.33 kWh of 'fuel'.
Example 3: smartphone battery capacity in kJ
4,000 mAh battery at 3.7V: energy = 4 × 3.7 = 14.8 Wh = 0.0148 kWh = 53,280 J ≈ 53 kJ.