Energy Units Guide
Understand joules, calories, kWh, and BTU — for food labels, electricity bills, and heating systems
Overview
Energy is the capacity to do work. It comes in many forms: mechanical, electrical, thermal, chemical, and nuclear. The SI unit is the joule (J), but we commonly use calories for food, kilowatt-hours (kWh) for electricity, and BTU for heating.
Understanding energy units helps with managing electricity bills, nutrition, and understanding physics.
Common Energy Units
J
Joule
SI unit of energy
Scientific standard
cal
Calorie
Food energy
1 cal = 4.184 J
kWh
Kilowatt-hour
Electricity bills
1 kWh = 3.6 MJ
BTU
British Thermal Unit
Heating/cooling
1 BTU = 1,055 J
Energy Unit Comparison
| Unit | In Joules | Primary Use |
|---|---|---|
| Joule (J) | 1 | Scientific calculations |
| Kilojoule (kJ) | 1,000 | Food labels (outside US) |
| Calorie (cal) | 4.184 | Chemistry |
| Kilocalorie (kcal) | 4,184 | Food energy (Calories) |
| Watt-hour (Wh) | 3,600 | Small electronics |
| BTU | 1,055 | US heating/cooling |
| Kilowatt-hour (kWh) | 3,600,000 | Electricity bills |
| Therm | 105,500,000 | Natural gas billing |
| Electronvolt (eV) | 1.6×10-19 | Particle physics |
Note: "Calories" on food labels are actually kilocalories (kcal). A 2,000 Calorie diet = 2,000 kcal = 8,368 kJ.
Real-World Energy Examples
Apple ~95 kcal (400 kJ)
Big Mac ~550 kcal (2,300 kJ)
LED Bulb (1 hour) ~10 Wh (36 kJ)
Phone Charge ~10-15 Wh
US Home (daily) ~30 kWh
Gallon of Gasoline ~120 MJ (33 kWh)
Lightning Bolt ~1 billion J (1 GJ)
Ton of TNT 4.184 GJ
Understanding Electricity Bills
How kWh Works
A kilowatt-hour = 1,000 watts used for 1 hour.
- 100W bulb × 10 hours = 1 kWh
- 1,000W microwave × 1 hour = 1 kWh
- 50W laptop × 20 hours = 1 kWh
Typical Costs (US)
- Average rate: $0.12-0.16 per kWh
- Running a 60W bulb 24/7 for a month ≈ $5
- Average US home: $120-150/month