How to use the Energy Converter
- Enter the energy quantity you have.
- Select its unit and the target unit.
- For nutrition, note that a food 'Calorie' is a kilocalorie — 1,000 small calories.
- For utility bills, kWh is the billing unit and therms appear on gas accounts.
- For heating and cooling equipment, BTU and BTU per hour are power, not energy — use the power converter for the latter.
How the calculation works
Everything converts through the joule, defined as one newton-metre of work or one watt-second. The joule is small in human terms, which is why almost every practical field adopted a multiple: kilojoules for food in most of the world, kilowatt-hours for electricity at 3.6 million joules each, and therms for gas at roughly 105.5 million joules.
The calorie is a persistent source of confusion because two units share the name. The small calorie is the energy to raise one gram of water by one degree Celsius, at 4.184 J under the thermochemical definition. The food Calorie printed on packaging is a kilocalorie, a thousand times larger. A 500 Calorie meal is therefore 500,000 small calories, or about 2,092 kJ — which is why European labels showing both kJ and kcal give figures differing by a factor of 4.184.
The BTU is the imperial counterpart: the energy to raise one pound of water by one degree Fahrenheit, about 1,055 joules. It survives in North American heating and cooling specifications, where an air conditioner might be rated at 12,000 BTU per hour — that figure is a rate of energy flow, not a quantity of energy, and equals one ton of refrigeration or about 3.5 kW.
energy_target = (E_source × J_per_source) ÷ J_per_target; 1 kcal = 4,184 J; 1 kWh = 3.6 × 10⁶ J; 1 BTU = 1,055.056 J; 1 therm = 100,000 BTUSource: BIPM SI Brochure 9th edition; ISO 80000-5; NIST Special Publication 811; IUPAC thermochemical calorie.
Worked example
An electric heater rated 2 kW runs for six hours and you want the cost and the equivalent in gas therms.
- Energy used: 2 kW × 6 h = 12 kWh.
- Enter 12 and select kilowatt-hours: 43.2 million joules.
- Convert to therms: 43,200,000 ÷ 105,505,585 = 0.409 therms.
- At 28p per kWh, the electricity costs 12 × 0.28 = £3.36.
12 kWh is 43.2 MJ or 0.41 therms — useful for comparing an electric heater directly against a gas alternative priced per therm.
Frequently asked questions
Is a Calorie the same as a calorie?+
No. The capitalised food Calorie is a kilocalorie — 1,000 small calories, or 4,184 joules.
How many joules in a kilowatt-hour?+
3,600,000. One watt-hour is 3,600 J, because a watt is a joule per second and an hour is 3,600 seconds.
Is BTU energy or power?+
BTU is energy. BTU per hour is power. Air conditioner ratings quoted as '12,000 BTU' almost always mean BTU per hour.
Why do food labels show both kJ and kcal?+
EU and UK regulations require kilojoules as the SI unit, while kilocalories remain what most consumers recognise, so both are printed.
Last reviewed September 1, 2026. We review this page whenever the underlying formula, tax year, published rate or standard changes.