Energy Converter
Convert energy units: joules, kilojoules, kilowatt-hours (kWh), calories, kilocalories (kcal/Calories), and British Thermal Units (BTU).
Reviewed by Richard Ross · Last updated April 2026
How Energy Converter works
Joule — the SI unit of energy
The joule is the SI unit of energy. 1 joule = 1 watt × 1 second. Common multiples: 1 kJ = 1,000 J; 1 MJ = 1,000,000 J.
kWh — electricity and gas billing
1 kWh = 3,600,000 joules (3.6 MJ). Energy bills in the UK are in kWh. 1 kWh of electricity at the current Ofgem cap costs approximately 24.5p.
Calories and kilocalories
The dietary "Calorie" (capital C) is actually a kilocalorie (kcal): 1 kcal = 1,000 cal = 4,184 J. Food energy is measured in kcal. A chocolate bar with 250 kcal provides 1,046 kJ of energy.
BTU and home heating
The British Thermal Unit (BTU) is the energy required to raise 1 pound of water by 1°F. Despite its British name, BTU is now almost exclusively used in the United States for boilers, air conditioners, and heating systems. In the UK, boiler output is rated in kilowatts (kW). A typical UK gas combi boiler outputs 24–35 kW (82,000–120,000 BTU/hour). When comparing boilers with American specs, convert: 1 kW = 3,412 BTU/hour.
Energy in everyday UK context
A typical UK home uses around 3,500 kWh of electricity and 12,000 kWh of gas per year (Energy Saving Trust figures). Running a kettle once uses roughly 0.1 kWh; a shower uses about 1.4 kWh; an electric oven for one hour uses around 2 kWh. Understanding kWh helps you interpret your smart meter readings and compare energy tariffs under the Ofgem price cap.
Precision and scale considerations
Energy spans an enormous range of magnitudes. A single heartbeat releases about 1 joule; a typical daily food intake is 8,400 kJ (2,000 kcal); the UK annual electricity consumption is around 300 TWh (3×10¹⁷ joules). When working at large scales, megajoules or kWh are more practical than joules. At the molecular level, electronvolts (eV) are used, but this converter focuses on everyday and engineering units.
Frequently asked questions
How many joules in a kWh?
1 kWh = 3,600,000 joules (3.6 MJ). Derived from 1,000 watts × 3,600 seconds.
What is the difference between a calorie and a kilocalorie?
A kilocalorie (kcal) = 1,000 calories (cal). Food labels and dietary advice use kcal, often written as "Calories" with a capital C to distinguish from the small calorie.
How many kWh is 1 BTU?
1 BTU = 0.000293071 kWh. Or 1 kWh = 3,412.14 BTU. BTU is commonly used for heating and air conditioning capacity in the US.
How many calories in a joule?
1 joule = 0.239006 calories. 1 calorie = 4.184 joules.
What does kWh mean on my electricity bill?
kWh (kilowatt-hour) is the unit of energy your electricity supplier bills you for. 1 kWh is the energy consumed by a 1,000-watt device running for 1 hour (e.g. a kettle, hair dryer, or microwave). The Ofgem price cap sets the maximum rate per kWh — currently around 24–25p for electricity.
How many kcal should I eat per day?
The NHS recommends approximately 2,000 kcal per day for women and 2,500 kcal for men as a rough average for maintaining weight. This equates to 8,368 kJ and 10,460 kJ respectively. Individual needs vary based on age, activity level, height, and weight — these figures are population averages.
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