Energy Converter

How many joules are in a kilowatt-hour?

1 kilowatt-hour equals 3,600,000 joules, or 3.6 MJ — the energy a 1,000 W appliance uses running for one hour. The kWh is the unit on every UK electricity bill because it turns an appliance's power rating into a billable amount of energy: a 2 kW kettle used for half an hour draws 1 kWh. Under the Ofgem price cap for 1 July to 30 September 2026, electricity averages about 26p per kWh (Ofgem unit rates, correct as of 2026-07-19), so that 1 kWh costs roughly 26 pence.

As a worked example you can check against the tool, type 1 into the kWh field: the converter returns 3,600,000 J, 3.6 MJ, 3,600 Wh, 860,420.7 cal, 860.4207 kcal and 3,412.128 BTU. The distinction people trip over is kW versus kWh — kW is power (the rate of energy use) while kWh is energy (the total used over time). Your meter measures kWh; your appliance's label states kW. For the closely related unit of power, use the power converter.

How do you convert calories to joules?

1 calorie equals 4.184 joules, but the "Calories" printed on UK food packaging are actually kilocalories, so 1 food Calorie (kcal) is 4,184 joules or 4.184 kJ. That factor of a thousand is why a 500 kcal meal is 2,092,000 joules — about 0.58 kWh, or roughly the energy to run a 60 W bulb for nine and a half hours. UK labels show both kJ and kcal, so this converter lets you move between the nutritional and scientific figures directly.

What is a BTU and how does it apply to UK boilers?

A British Thermal Unit is the energy needed to raise one pound of water by 1°F, and 1 BTU equals 1,055.06 joules. Boilers, air-conditioning units and heat pumps are still often rated in BTU/hr, so a typical UK combi boiler at 24,000–35,000 BTU/hr is putting out 7–10 kW. The quick rule is to divide a BTU/hr figure by 3,412 to get kilowatts, which lets you compare an imported air-conditioner's BTU rating against a UK boiler's kW output on the same scale.

How much energy does a UK household use, and how is gas billed?

Ofgem's typical (medium) domestic consumption values from 1 July 2026 are 2,500 kWh of electricity and 9,500 kWh of gas a year, down from the previous 2,700 and 11,500 (Ofgem TDCV, correct as of 2026-07-19). These medians are what the headline price cap is calculated against. Gas is metered by volume in m³ or ft³ and converted to kWh for billing using the calorific value of the gas — about 11.2 kWh per m³ — before the per-kWh unit rate is applied.

What is an electronvolt used for?

An electronvolt (eV) is the energy an electron gains crossing a 1 volt potential difference, equal to 1.60218 × 10⁻¹⁹ joules — an almost unimaginably tiny amount next to a kilowatt-hour. It is the natural currency of atomic, molecular and particle physics: chemical bond energies run to a few eV, visible-light photons are around 2–3 eV, and particle accelerators reach billions of eV (GeV). The converter includes it so a physics or chemistry figure in eV translates straight into joules for comparison.

The huge span between an electronvolt and a megawatt-hour — more than 25 orders of magnitude — is exactly why a single converter is useful: it holds atomic, nutritional, domestic and industrial energy on one scale. The foot-pound (1.35582 J), the imperial unit of mechanical work, sits in the middle and shares its dimension with torque, which is why 1 ft·lbf and 1 lbf·ft carry the same number. For that rotational cousin, see the torque converter.