Calories, Joules and Kilowatt-Hours Explained
Energy is one quantity with units from three different worlds: physics, nutrition and electricity bills. The Energy Converter moves between joules, calories and kilowatt-hours.
The calorie that is not a calorie
This is the one worth getting straight. A calorie in physics is the energy needed to heat one gram of water by one degree Celsius - about 4.184 joules, a tiny amount.
The "calorie" on a food label is a kilocalorie: a thousand of those. So a 2,000 calorie diet is really 2,000 kilocalories, or 8.4 million joules.
The convention is to write the food one with a capital C, or as kcal, but packaging is inconsistent and plenty of labels just say calories. European labels usually show both kJ and kcal, which is why the kJ number looks alarming - a chocolate bar at 1,000 kJ is 239 kcal.
Joules, the actual unit
A joule is one newton acting over one metre. It is the SI unit and, like the pascal, it is too small for everyday quantities, so real figures come in kilojoules or megajoules.
A useful anchor: lifting a kilogram by one metre takes about 10 joules. Which tells you how much energy food actually contains - a single kilocalorie would lift that kilogram over 400 metres.
Kilowatt-hours, the billing unit
A kilowatt-hour is a thousand watts drawn for an hour, which is 3.6 million joules. It is what your electricity meter counts, because joules would make the numbers absurd.
The distinction to hold onto is power versus energy. A watt is a rate - how fast energy is used. A kilowatt-hour is an amount. A 2 kW heater running for 30 minutes uses 1 kWh, the same as a 1 kW heater for an hour. The Power Converter deals with the rate; this one deals with the total.
Some rough figures: a kettle is about 3 kW, so boiling it for three minutes is 0.15 kWh. An LED bulb is 10 watts, so a hundred hours is 1 kWh. A fridge uses roughly 1 to 1.5 kWh a day. An electric car does about 4 miles per kWh.
Common questions
How many calories in a kilowatt-hour?
About 860 kilocalories - roughly a substantial meal, which is a striking comparison between what a body runs on and what a house does.
Why are European labels in kilojoules?
Because kJ is the SI unit and the regulation requires it. The kcal figure sits beside it because that is what people understand.
Does burning calories mean heat?
Largely, yes. Most of the energy a body uses ends up as heat, which is why a crowded room warms up.
What is a BTU?
A British Thermal Unit, about 1,055 joules, still used for heating and air conditioning in the US.
Why is my gas bill in kWh when gas is sold by volume?
Because the meter measures cubic metres and the bill converts that to energy, using a calorific value for the gas supplied. Two fuels of the same volume do not carry the same energy, so billing by energy is the fairer measure.
Got an energy figure to convert?