About the Energy Converter
This energy converter spans nine units and more than twenty orders of magnitude, from the electronvolt used in particle physics up to the kilowatt hour on your electricity bill. It covers joules and kilojoules, calories and food kilocalories, watt hours and kilowatt hours, British thermal units, foot-pounds and electronvolts. All of them measure the same physical quantity, which is why a food label, a heating system specification and a utility statement can be placed on a common footing — something that is otherwise surprisingly hard to do.
How to use the Energy Converter
- 1
Enter the energy figure in the Value box.
- 2
Choose the unit it is given in from the From list.
- 3
Choose the unit you want from the To list.
- 4
Read the result and use the table to compare the same amount of energy across all nine units.
What people use it for
Reading nutrition labels
EU labels give both kJ and kcal. A 250 kcal snack is 1,046 kJ, and a 2,000 kcal daily intake is 8,368 kJ.
Understanding an electricity bill
A 2 kW heater running for three hours uses 6 kWh, which is 21.6 megajoules. Multiplying kWh by your unit rate gives the actual cost.
Sizing heating and cooling
Air conditioners are rated in BTU per hour in some markets and kilowatts in others. A 12,000 BTU/h unit is about 3.5 kW of cooling capacity.
The joule, and how the everyday units relate to it
The joule is the SI unit of energy: the work done when a force of one newton moves an object one metre. It is small, roughly the energy needed to lift an apple a metre, so practical units are much larger multiples. A watt is one joule per second, so a watt hour is 3,600 joules and a kilowatt hour is exactly 3,600,000 joules, or 3.6 megajoules — the single most useful conversion here, since it links your electricity meter to physics. The calorie used in this tool is the thermochemical calorie of exactly 4.184 joules, historically the energy needed to warm one gram of water by one degree Celsius. The kilocalorie is therefore 4,184 joules, and it is what nutrition labels mean by a Calorie with a capital C, so a 2,000 Calorie diet is 2,000 kcal or 8.368 megajoules. The British thermal unit, the energy to warm one pound of water by one degree Fahrenheit, is about 1,055.06 joules in its international table form, which makes 1 kilowatt equal to roughly 3,412 BTU per hour. The foot-pound, at 1.3558 joules, is the imperial mechanical work unit and still appears in torque and ballistics figures.
The electronvolt and the scale of things
At the opposite extreme sits the electronvolt, the kinetic energy an electron gains when accelerated through a potential difference of one volt. Since the 2019 SI revision the elementary charge is fixed at exactly 1.602176634 × 10⁻¹⁹ coulombs, so an electronvolt is exactly that many joules. It is used wherever joules would be absurd. Visible light photons carry between roughly 1.6 and 3.3 eV, chemical bonds sit in the range of a few eV, the ionisation energy of hydrogen is 13.6 eV, and the Large Hadron Collider accelerates protons to around 6.8 tera-electronvolts each — which is still only about a microjoule, comparable to a flying mosquito but concentrated into a single particle. Placing the extremes side by side makes the span vivid: one kilowatt hour is roughly 2.2 × 10²⁵ electronvolts. A useful piece of intuition at human scale is that a person eating 2,000 kcal a day is averaging close to 100 watts of continuous power output, about the same as an old incandescent light bulb, which is why a crowded room warms up so noticeably.
Tips
- One kilowatt hour is exactly 3.6 megajoules — the anchor point for any energy conversion involving electricity.
- The Calorie on food packaging is a kilocalorie; the difference is a factor of 1,000 and it is a common source of error.
- Watts measure power, not energy — multiply by hours to obtain the energy actually consumed.