Article — Energy Conversion Calculator
Energy conversion: joules, calories, BTU, kWh, electronvolts
Energy conversion translates a value measured in one unit (calories, joules, kWh, BTU, electronvolts) into another. All nine units in this calculator route through the joule, the SI base. The conversion factors are exact or defined: 1 cal = 4.184 J, 1 kWh = 3,600,000 J, 1 BTU = 1055.06 J, 1 eV = 1.602176634×10−¹⁹ J. These are physical definitions, not measurements.
Energy is the same physical quantity in every form — chemical, electrical, kinetic, thermal, radiation. The conservation law says energy cannot be created or destroyed. The unit it is reported in is purely a matter of convention and audience.
What is energy conversion?
In the unit-conversion sense, energy conversion is arithmetic: multiply by an exact factor to swap units. In the physics sense, energy conversion describes the transformation of energy from one form to another — chemical to thermal in a flame, electrical to kinetic in a motor, kinetic to thermal in friction. This article is about the first sense, but the second context is what makes the conversions useful.
A calorie is just as much a unit of energy as a joule is. They differ in size, in historical origin, and in the field of work that uses them, but they describe the same physical thing. The calculator lets you move between them without dropping precision.
Joule: the energy conversion hub
The joule (J) is the SI unit of energy. Defined as the work done by a 1-newton force acting over 1 metre, or equivalently 1 watt of power sustained for 1 second. In base units: 1 J = 1 kg·m²/s².
James Prescott Joule’s 1843 paddle-wheel experiments proved that mechanical work and heat are interchangeable, and the joule was named for him in 1882. The CGPM formally adopted it as the SI energy unit in 1948.
1 J = 1 N·m = 1 W·s1 J = 1 kg·m²/s²1 kJ = 1000 J1 MJ = 10⁶ JCalorie vs kilocalorie energy
The calorie is a thermal unit predating SI. Originally defined as the energy to raise 1 g of water by 1 °C, it was standardised by the thermochemical community as exactly 4.184 J. The kilocalorie (kcal) is 1000 calories.
Confusion arises on food labels. US nutrition labelling uses “Calorie” with a capital C to mean kilocalorie. So a 2000-Calorie diet is 2,000,000 small-calorie units = 8.4 MJ:
1 Cal = 1 kcal = 1000 cal2000 Cal/day = 2000 kcal/day= 8.368 MJ/day = 96.85 W average powerThat 96.85 W is your basal-plus-active metabolic rate averaged over 24 hours — comparable to a bright LED lamp. EU labels typically list both kJ and kcal to remove ambiguity. US labels still use Calorie alone.
The historical definition of the calorie required specifying water temperature because water’s specific heat varies very slightly with temperature. Five flavours existed: the 4 °C calorie, the 15 °C calorie, the 20 °C calorie, the thermochemical calorie (4.184 J), and the IT (International Table) calorie (4.1868 J). Modern chemistry uses the thermochemical value almost universally.
Kilowatt-hour energy conversion
The kilowatt-hour (kWh) is the unit on your electricity bill. It equals 1000 W of power sustained for 1 hour, which is 3,600,000 J or 3.6 MJ. The kWh is awkward in pure physics — you would much rather use J or MJ — but it survives because utilities have used it since Edison’s era and consumers are used to it.
Typical kWh usage figures:
- 10 W LED, 24h: 0.24 kWh = 864 kJ
- Electric kettle (2 kW), 3 min: 0.10 kWh
- Tumble dryer cycle: 2–5 kWh
- EV charging, 60 kWh battery: 60 kWh = 216 MJ
- Average US household/day: ~30 kWh = 108 MJ
- Litre of petrol: ~9.4 kWh of chemical energy
Electronvolt energy scales
For atomic and particle physics, the joule is far too large. One eV equals 1.602176634×10−¹⁹ J — tiny on a human scale but natural for atomic processes.
An electron accelerated through a potential difference of 1 volt gains 1 eV of kinetic energy. Hydrogen’s ionisation energy is 13.6 eV. Visible-light photons carry 1.7–3.1 eV. The Large Hadron Collider accelerates protons to 6.5 TeV (6.5×10¹² eV), which sounds enormous but works out to only 1.04 microjoules per proton.
1 eV = 1.602176634×10−¹⁹ J1 keV = 1000 eV (X-ray range)1 MeV = 10⁶ eV (gamma rays)1 GeV = 10⁹ eV (proton rest mass: 0.938 GeV)1 TeV = 10¹² eV (LHC range)BTU and other imperial energy units
The British Thermal Unit (BTU) measures the energy needed to raise 1 lb of water by 1 °F. The ISO value is 1 BTU = 1055.06 J. BTUs are the standard for US heating and cooling: a 12,000 BTU/h air conditioner moves heat at 3.517 kW.
The foot-pound-force (ft·lbf) is the imperial work unit: 1 ft·lbf = 1.3558 J. Used in mechanical specs (engine torque, tool ratings) in the US.
The erg is the CGS energy unit, with 1 erg = 10−⁷ J. Largely obsolete except in astrophysics, where cosmic energies happen to be reasonable-sized erg numbers. A supernova releases about 10⁵¹ erg (10⁴⁴ J).
When sizing an air conditioner, divide BTU/h by 3412 to get kW. A 12,000 BTU/h unit = 3.52 kW; an 18,000 BTU/h unit = 5.27 kW. EU sellers usually advertise the kW number; US sellers advertise BTU/h. They describe the same cooling capacity.
Common energy conversion mistakes
Three errors cover most energy-unit confusion:
The kilowatt is power (J/s); the kilowatt-hour is energy. A 1 kW kettle uses 1 kWh in an hour. The BTU is energy; BTU/h is power. Watts is to joules as miles-per-hour is to miles. Mixing the two is the most common mistake in HVAC and electrical specs.
- cal vs Cal: lowercase = gram calorie, capital = kcal (food)
- kWh vs kW: energy vs power; off by a time factor
- BTU vs BTU/h: same energy/power confusion in imperial form
Energy units in real-world context
Putting a few values side by side helps build intuition for which energy unit fits which scale.
A AA alkaline battery stores about 10 kJ — enough to power a 100 mW LED nightlight for 28 hours. A litre of petrol contains about 34 MJ of chemical energy, of which a typical internal combustion engine extracts roughly 30% as mechanical work (the rest leaves as heat). One kWh of electricity costs the consumer 10–40 US cents depending on country; one MJ costs proportionally less (1 kWh = 3.6 MJ).
At cosmic scales the numbers explode. A typical category 5 hurricane releases about 10¹ J of latent heat per day, comparable to all global electricity generation. The 1908 Tunguska event released roughly 4×10¹⁶ J = 40 PJ, or about 12 megatons of TNT. The Krakatoa explosion of 1883: about 8×10¹⁷ J = 200 megatons. The Sun radiates 3.8×10²⁶ W — that is 3.8×10²⁶ J every second, equivalent to about 10 billion Tunguskas per second. Energy units are a tool for keeping numbers like these tractable for human reading.