Energy Unit Conversion — Complete Guide
Energy is a fundamental physical quantity that describes the capacity of a system to do work. The SI base unit is the joule (J), but different fields use different units: calories and kilocalories in nutrition, kilowatt-hours in electricity billing, BTU in heating and cooling systems, and electronvolts in nuclear and particle physics. Our energy converter supports instant conversion between 12 units and covers everything from atomic-scale electronvolts to large-scale tonnes of oil equivalent.
Joules to Calories — and Back
The joule is the SI standard: 1 J equals the work done by a force of 1 newton over 1 metre, or equivalently 1 watt of power for 1 second. The calorie was historically defined as the heat needed to raise 1 gram of water by 1 °C, giving the exact relation 1 cal = 4.184 J. In nutrition, the kilocalorie (kcal) is the working unit — what food labels call a "Calorie" (capital C) is actually 1 kcal = 4,184 J = 4.184 kJ. To convert kcal to kJ, multiply by 4.184; to convert kJ to kcal, divide by 4.184.
Kilowatt-hours — How Electricity is Measured
The kilowatt-hour (kWh) is the practical unit for electrical energy and the one that appears on electricity bills worldwide. By definition, 1 kWh is the energy consumed by a 1 kW device running for exactly one hour, which works out to 3,600,000 J = 3.6 MJ = 860.4 kcal. To convert kWh to MJ, multiply by 3.6. To convert MJ to kWh, divide by 3.6. A useful rule of thumb: 1 litre of gasoline holds roughly 8.9 kWh of chemical energy.
BTU — Heating, Cooling, and HVAC Systems
The British Thermal Unit (BTU) remains the standard in the US heating and cooling industry. One BTU is the heat required to raise 1 pound of water by 1 °F, equal to 1,055.06 J or about 252 calories. Air conditioner and heat pump capacity is rated in BTU per hour (BTU/h): a common 12,000 BTU/h unit delivers roughly 3.5 kW of cooling. To convert kWh to BTU, multiply by 3,412. To convert BTU to kWh, multiply by 0.000293.
Comparing Fuels by Energy Content
When comparing energy sources, megajoules (MJ) or the tonne of oil equivalent (toe) provide a neutral common scale. One litre of petrol contains approximately 32 MJ, one cubic metre of natural gas approximately 34 MJ, and one kilogram of coal approximately 29 MJ — all significantly more energy-dense than the 3.6 MJ in 1 kWh of electricity. The toe, equal to 41.868 GJ or 11,630 kWh, is widely used in national energy statistics and international comparisons of energy production and consumption.