Energy converter

Every energy unit, one page

Pick any two units above. The conversions people look up most often have dedicated pages with reference tables, linked below.

The factors

The food label Calorie is a kilocalorie

A capital-C Calorie is 1000 lower-case calories. Food labelling uses the large one throughout, so “250 Calories” on a packet means 250 kcal — which is 250,000 of the calorie a physicist means.

Almost nothing outside nutrition uses the small calorie any more, so in practice “calories” on food means kilocalories, and the two are only distinguished when it matters.

EU and Australian labels are required to print kilojoules; UK, US, Taiwanese and Hong Kong labels print kilocalories; many print both. See kcal to kJ.

Energy in food

These are the Atwater factors, and they are averages rather than measurements of any particular food — which is why a label’s stated energy and the sum of its macronutrients rarely agree exactly.

Energy is not power

The distinction that causes the most confusion here:

A kilowatt-hour is a kilowatt sustained for an hour. A 2 kW heater run for 3 hours uses 6 kWh — and an electricity bill charges for kWh, never for kW. See the power converter.

A day’s food against a day’s electricity

Putting the two on the same scale is the most useful thing this converter does:

A person eats about 2.3 kWh a day. A household uses four times that in electricity alone, and a car uses nearly twice a person’s daily intake to travel 100 km.

Reference energies

A litre of petrol holds about nine and a half kilowatt-hours, which is why a 60 kWh battery stores roughly the energy of six litres of fuel — and why the comparison only makes sense alongside the fact that an electric motor wastes far less of it.