Energy Unit Converter
Energy is the capacity to do work. Different scientific and practical fields use different energy units: physicists use joules and electron-volts, the power industry uses kilowatt-hours, nutritionists use kilocalories, and the HVAC industry uses BTUs. This converter provides instant conversion between joules (J), kilojoules (kJ), kilowatt-hours (kWh), thermochemical calories (cal), kilocalories (kcal), British Thermal Units (BTU), and electron-volts (eV). All factors follow NIST SP 811.
Energy conversion factors (referenced to joule)
1 J = 0.001 kJ = 2.7778e-7 kWh = 0.239006 cal = 2.39006e-4 kcal = 9.4782e-4 BTU = 6.2415e18 eV
All conversions route through the joule. The thermochemical calorie (4.184 J exactly) is used here, as defined by NIST. The BTU used is the International Table BTU (1,055.05585 J).
Energy unit applications
- Joule (J): SI base unit. Used in physics, chemistry, and electrical engineering.
- kWh: Electricity billing, battery capacity, and solar energy generation.
- cal / kcal: Nutrition (kcal), chemistry calorimetry (cal).
- BTU: HVAC heating/cooling loads, natural gas billing, and boiler ratings in the US.
- eV: Atomic and nuclear physics, semiconductor band gaps, radiation dosimetry.
Energy unit converter: frequently asked questions
How many joules are in a kilowatt-hour?
One kilowatt-hour (kWh) equals exactly 3,600,000 joules (3.6 MJ). This is because 1 kW = 1,000 J/s and 1 hour = 3,600 s, so 1 kWh = 1,000 x 3,600 = 3,600,000 J.
What is the difference between a calorie and a food Calorie?
A small calorie (cal) is the energy to raise 1 gram of water by 1 degree Celsius, equal to 4.184 J. A food Calorie (written with a capital C, or kcal) equals 1,000 small calories, or 4,184 J. Nutrition labels always use the food Calorie (kcal).
What is a BTU and when is it used?
A British Thermal Unit (BTU) is the energy needed to raise 1 pound of water by 1 degree Fahrenheit. One BTU equals 1,055.06 J. BTUs are widely used in the US for heating and cooling equipment specifications (HVAC) and natural gas billing.
What is an electron-volt used for?
The electron-volt (eV) is the energy gained by one electron moving through a potential difference of 1 volt. It equals 1.602176634e-19 J. eV is the standard energy unit in atomic, nuclear, and particle physics because other units are far too large for atomic-scale energies.
How many BTUs are in a kWh?
One kilowatt-hour equals 3,412.14 BTU. This relationship is important in comparing electricity costs to natural gas costs, since gas is typically billed in BTU or therms (100,000 BTU) and electricity in kWh.
Official sources
- NIST Special Publication 811: Guide for the Use of the International System of Units (SI).
- NIST CODATA Fundamental Constants: Electron-volt definition.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.