How many joules are in a kilowatt hour?

Exactly 3,600,000 joules, or 3.6 megajoules. A kilowatt hour is 1000 watts sustained for 3600 seconds, and one watt-second is one joule.

Is a food calorie the same as a calorie?

No. The calorie on a nutrition label is a kilocalorie, equal to 1000 small calories or 4184 joules. Physics uses the small calorie. This is why a chocolate bar shows perhaps 250 kcal alongside 1046 kJ.

How do I convert kJ to calories?

Divide kilojoules by 4.184 to get kilocalories, the figure most people mean by calories. A 2000 kJ meal is about 478 kcal.

What is the difference between kW and kWh?

A kilowatt is a rate of energy use; a kilowatt hour is an amount of energy. A 3 kW heater running for two hours uses 6 kWh. Energy suppliers bill in kWh.

How much energy does a typical household use?

A UK home uses roughly 2700 kWh of electricity a year, though this varies widely with heating type, home size and occupancy. That is about 7.4 kWh per day, or 26.6 MJ.

Energy Converter - kWh to Joules, Calories, kJ & BTU

Energy Converter Tool

Value

From

Result

Kilojoule (kJ)

Energy versus power

These are routinely confused. Power is a rate, measured in watts. Energy is the total delivered over time, measured in joules or kilowatt hours. A 2000 W kettle draws power at 2000 watts; run it for six minutes and it consumes 0.2 kWh of energy. Your electricity bill charges for energy, not power.

The relationship is simply energy = power × time. One watt sustained for one second is one joule, so a kilowatt hour is 1000 W × 3600 s = 3,600,000 joules.

Food labels add their own wrinkle. The “calorie” on a nutrition panel is really a kilocalorie: 1000 small calories, or 4184 joules. UK and EU labels show both kJ and kcal, which is why the kJ figure is always about 4.2 times larger. To work out appliance running costs, use the electricity consumption calculator; for rates rather than totals, use the power converter.

Quick reference

Common energy conversions

Conversion	Result
1 kWh	3,600,000 joules (3.6 MJ)
1 kWh	3412.14 BTU
1 Wh	3600 joules
1 kcal (food calorie)	4184 joules (4.184 kJ)
1 calorie	4.184 joules
1 BTU	1055.06 joules
1 MJ	0.2778 kWh
1 kJ	0.239 kcal
100 kcal snack	418.4 kJ
1 electronvolt	1.602177 × 10⁻¹⁹ joules

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Questions

Frequently Asked Questions

How many joules are in a kilowatt hour?: Exactly 3,600,000 joules, or 3.6 megajoules. A kilowatt hour is 1000 watts sustained for 3600 seconds, and one watt-second is one joule.
Is a food calorie the same as a calorie?: No. The calorie on a nutrition label is a kilocalorie, equal to 1000 small calories or 4184 joules. Physics uses the small calorie. This is why a chocolate bar shows perhaps 250 kcal alongside 1046 kJ.
How do I convert kJ to calories?: Divide kilojoules by 4.184 to get kilocalories, the figure most people mean by calories. A 2000 kJ meal is about 478 kcal.
What is the difference between kW and kWh?: A kilowatt is a rate of energy use; a kilowatt hour is an amount of energy. A 3 kW heater running for two hours uses 6 kWh. Energy suppliers bill in kWh.
How much energy does a typical household use?: A UK home uses roughly 2700 kWh of electricity a year, though this varies widely with heating type, home size and occupancy. That is about 7.4 kWh per day, or 26.6 MJ.