Energy appears in many forms — mechanical, heat, electricity, food — and each domain uses different units. They all measure the same thing: energy.
The Joule: Universal Energy Unit
1 Joule = 1 Newton-meter. 1 kJ = 1,000 J. All forms of energy can be expressed in Joules.
Calories: Food and Heat
The dietary Calorie (kcal) is 1,000 small calories. 1 Cal = 1 kcal = 4,184 J. A food label showing "200 Calories" means 836.8 kJ.
Kilowatt-Hours and BTUs
1 kWh = 3,600,000 J = 3.6 MJ. Your electricity bill uses kWh. 1 BTU ≈ 1,055 J. A typical room AC is 12,000 BTU/h ≈ 3.5 kW.
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Frequently Asked Questions
How many joules are in a kilowatt-hour?
1 kWh = 3,600,000 J = 3.6 MJ. Because 1 kW = 1,000 J/s × 3,600 s = 3,600,000 J.
What is the difference between calorie and Calorie?
A dietary Calorie (kcal) = 1,000 small calories (cal). Food labels use Calories. 1 Cal = 4,184 J.