Understanding energy
One quantity, many habits.
Joules for physics, calories for food, kilowatt hours for the electricity bill. They all measure the same thing.
The joule, the watt-hour, the calorie.
The joule is the SI unit — the energy a one-newton force expends moving one metre. Almost every other energy unit is a fixed number of joules: a calorie is the energy needed to raise a gram of water by one degree Celsius (4.184 joules, by definition); a kilowatt hour is the energy a one-kilowatt appliance burns in an hour (3.6 million joules); a BTU comes from heating a pound of water (about 1055 joules).
1 cal ≡ 4.184 J · 1 kWh ≡ 3 600 000 J · 1 BTU ≈ 1055.06 J
Calorie vs kilocalorie.
A nutritional "calorie" on a food label is actually a kilocalorie — a thousand small calories. The capitalisation is a clue; the confusion is endemic. A 200 Cal granola bar is 200 kcal is 836 800 J of chemical energy.
Practical equivalences
- 1 kcal = 1000 cal = 4184 J
- 1 Wh = 3600 J · 1 kWh = 3.6 MJ
- 1 BTU ≈ 1055.06 J ≈ 0.252 kcal
- 1 ft·lb ≈ 1.356 J
Three worked conversions.
1 kWh to joules
1 kWh = 3 600 000 J
Multiply by the factor.
1 × 3 600 000 = 3 600 000
= 3 600 000 J = 3.6 MJ
500 kcal to kilojoules
1 kcal = 4.184 kJ
A kilocalorie is bigger than a kilojoule, so the count grows — multiply.
500 × 4.184 = 2092
= 2092 kJ
10 000 BTU to kWh
1 BTU ≈ 1055.06 J · 1 kWh = 3 600 000 J
Convert through joules.
10 000 × 1055.06 ÷ 3 600 000 ≈ 2.93
= 2.93 kWh
A note on definitions.
There are several "calories" in circulation — the thermochemical (4.184 J), the IT (4.1868 J), and the 15 °C (4.1855 J). The converter uses the thermochemical value, which is the one used on food labels. For chemistry papers, check the source.
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