Conversion Reference Table
| Unit | Symbol | Equivalent |
|---|---|---|
| Joule (J) | J | SI base unit: 1 kg·m²/s² |
| Kilojoule (kJ) | kJ | 1 kJ = 1,000 J |
| Kilowatt-hour (kWh) | kWh | 1 kWh = 3,600,000 J |
| Calorie (small cal) | cal | 1 cal = 4.184 J (exact) |
| Kilocalorie (food Cal) | kcal | 1 kcal = 4,184 J = 1 food Calorie |
| BTU | BTU | 1 BTU = 1,055.06 J |
| Electronvolt | eV | 1 eV = 1.602×10⁻¹⁹ J |
| Foot-pound | ft·lbf | 1 ft·lbf = 1.35582 J |
| Therm (US) | therm | 1 therm = 105,480,400 J |
Frequently Asked Questions
What is a food Calorie vs a calorie?
A food Calorie (written with capital C) equals 1 kilocalorie (kcal) = 4,184 joules. The small calorie (cal) is 1/1000 of this. When nutrition labels say '200 Calories', they mean 200 kcal. This can be confusing — a candy bar with 200 'Calories' actually contains 200,000 small calories.
How many joules are in a kilowatt-hour?
1 kilowatt-hour (kWh) = 3,600,000 joules (3.6 MJ). A kWh is the energy used by a 1,000-watt device running for 1 hour. Electricity bills are charged in kWh — in the US, the average household uses about 900 kWh/month.
What is a BTU?
A British Thermal Unit (BTU) is the energy needed to raise 1 pound of water by 1°F. It equals 1,055.06 joules. BTUs are used to rate heating and air conditioning systems — a typical residential AC unit is 12,000 BTU/hour (1 ton of cooling).
How is energy different from power?
Energy is the total amount of work done (joules, kWh). Power is the rate of energy transfer (watts = joules per second). A 100-watt light bulb running for 10 hours uses 1,000 watt-hours = 1 kWh of energy.