Energy Units Explained: Calories, Joules, kWh, and BTU
Understand energy units used in nutrition, physics, electricity bills, and heating.
Energy is measured in many different units depending on the context. Calories appear on food labels, kilowatt-hours on electricity bills, BTUs in heating specifications, and joules in physics textbooks. Understanding how these units relate helps you make informed decisions about diet, energy consumption, and more.
The Main Energy Units
Joule (J) and Kilojoule (kJ)
The SI unit of energy. One joule is the energy needed to move a 1-Newton force through 1 meter. Kilojoules are commonly used on food labels outside the US.
Calorie and Kilocalorie (cal, kcal)
In nutrition, a Calorie (capital C) actually refers to a kilocalorie (1,000 small calories). When food labels say 200 Calories, they mean 200 kilocalories or 837 kilojoules.
Kilowatt-hour (kWh)
The standard unit for electricity billing. One kWh is the energy consumed by a 1,000-watt appliance running for one hour. The average US household uses about 900 kWh per month.
BTU (British Thermal Unit)
Used in heating, cooling, and HVAC systems, particularly in the US. One BTU is the energy needed to raise one pound of water by one degree Fahrenheit.
Key Conversions
- 1 kcal = 4.184 kJ = 4,184 J
- 1 kWh = 3,600,000 J = 860 kcal
- 1 BTU = 1,055 J = 0.252 kcal
- 1 kWh = 3,412 BTU
Practical Energy References
- A banana: ~105 kcal (440 kJ)
- Running 1 mile: ~100 kcal burned
- LED light bulb (10W, 1 hour): 0.01 kWh
- Charging a smartphone: ~0.01 kWh
- Electric car per mile: ~0.3 kWh
- Air conditioner (1 hour): 1-3 kWh
Need to convert units?
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