Energy & Power Converters

42 converters, all running in your browser.

The joule is the SI unit of energy, and every other unit here exists because a particular industry standardised on something else before the SI did.

The calorie is the worst offender. A calorie is the energy to raise one gram of water by one degree Celsius — about 4.184 joules. But the "calorie" on a nutrition label is a kilocalorie, a thousand of those. A 500-calorie meal is 500 kcal, or roughly 2.1 million joules. Some countries write it as Cal with a capital C to mark the difference; most do not. These converters treat calories and kilocalories as separate units to avoid the ambiguity.

The kilowatt hour is an energy unit despite sounding like a power unit: it is one kilowatt sustained for one hour, or 3.6 million joules. It survives because electricity is metered that way.

The BTU is the energy to raise one pound of water by one degree Fahrenheit, and it persists in heating and air-conditioning specifications almost everywhere, including in countries that are otherwise fully metric.

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Conversion factors

From	To	Multiply by
Joules	Kilojoules	0.001
Joules	Calories	0.23900574
Joules	Kilocalories	2.3901e-4
Joules	Watt Hours	2.7778e-4
Joules	Kilowatt Hours	2.7778e-7
Kilojoules	Joules	1000
Kilojoules	Calories	239.00574
Kilojoules	Kilocalories	0.23900574
Kilojoules	Watt Hours	0.27777778
Kilojoules	Kilowatt Hours	2.7778e-4
Calories	Joules	4.184
Calories	Kilojoules	0.004184
Calories	Kilocalories	0.001
Calories	Watt Hours	0.0011622222
Calories	Kilowatt Hours	1.1622e-6
Kilocalories	Joules	4184
Kilocalories	Kilojoules	4.184
Kilocalories	Calories	1000
Kilocalories	Watt Hours	1.1622222
Kilocalories	Kilowatt Hours	0.0011622222

Reading a Nutrition Label

The kilocalorie confusion is worth stating twice because it changes numbers by a thousand. A label saying "250 calories" means 250 kilocalories — about 1,046 kilojoules. Labels in the EU and Australia print kilojoules alongside, which is why the same biscuit shows 250 in one country and 1,046 in another.

A rough conversion for reading either: kilojoules ÷ 4.2 gives kilocalories.

Household Energy

Electricity is billed in kilowatt hours, and the appliance rating tells you the rate:

Appliance	Rating	Cost per hour at 30p/kWh
LED bulb	10 W	0.3p
Laptop	60 W	1.8p
Fridge	150 W average	4.5p
Electric heater	2,000 W	60p
Electric shower	9,000 W	£2.70

The spread is what makes the advice about heating matter more than the advice about standby power: an electric heater costs two hundred times what a bulb does.

Gas is billed in kWh in some countries and therms or cubic metres in others, which is why the comparison between heating fuels usually needs a conversion before it can be made at all.

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