Energy Conversion

Supported units

Every unit below is a pure scale of the joule, so converting between any two is a single division of their factors.

Most are exact by definition — the thermochemical calorie is exactly 4.184 J under ISO 80000-5, and the electronvolt is exact because the elementary charge is a fixed SI constant (NIST SP 811). The BTU is the exception: the figure here uses the IT calorie definition, and other BTU definitions land anywhere between 1054 and 1060 J — a difference worth knowing about before quoting one to three decimal places.

Energy units

Watt-hour (Wh)

The energy a one-watt device uses in one hour, which is exactly 3 600 joules. It is the natural unit for electrical consumption because power ratings are given in watts and billing periods in hours.

Electrical consumption

Energy drawn by light bulbs, computers and small appliances.

Batteries

Capacity of laptop, phone and power-tool batteries.

Metering

The quantity utility meters accumulate.

Kilowatt-hour (kWh)

One thousand watt-hours, or 3.6 megajoules. This is the unit on virtually every electricity bill in the world, and the unit electric vehicle and home battery capacities are quoted in.

Electricity billing

The billed quantity on domestic and commercial energy bills.

Consumption tracking

Household and business energy use for heating, lighting and appliances.

Renewable energy

Output of rooftop solar arrays and small wind turbines.

Megawatt-hour (MWh)

One thousand kilowatt-hours, or one million watt-hours. Wholesale electricity is traded by the megawatt-hour, and generation output is reported in it.

Power generation

Electricity produced by a plant or wind farm over a period.

Grid operation

Tracking energy distribution and demand across a network.

Energy trading

The contract unit in wholesale electricity markets.

Gigawatt-hour (GWh)

One thousand megawatt-hours, or one million kilowatt-hours. National and regional generation statistics are usually tallied in gigawatt-hours.

Power plants

Annual output of large generating stations and solar parks.

National statistics

Country-level electricity generation and consumption.

Large consumers

Energy use of cities, data centres and heavy industry.

Electronvolt (eV)

The kinetic energy an electron gains crossing a potential difference of one volt: exactly 1.602 176 634 x 10^-19 joules, since the elementary charge is a fixed SI constant. It is the working energy unit of atomic and particle physics.

Particle physics

Energies of particles in accelerators and collisions.

Atomic and molecular physics

Electron energy levels and transition energies.

Solid-state physics

Semiconductor band gaps — silicon is about 1.1 eV.

Mega electronvolt (MeV)

One million electronvolts, or 1.602 176 634 x 10^-13 joules. Nuclear binding energies and the rest masses of light particles are naturally sized in MeV — an electron is 0.511 MeV/c².

Nuclear physics

Binding energies and the energy released by fission and fusion.

Particle physics

Rest masses of electrons, protons and mesons.

Medical physics

Beam energies in radiotherapy and PET imaging.

British Thermal Unit (BTU)

The heat needed to raise one pound of water by one degree Fahrenheit — roughly 1 055 joules. There is no single BTU: depending on the water temperature the definition assumes, values range from about 1 054 to 1 060 J. This converter uses the International Table BTU of 1 055.055 853 J.

Heating and cooling

Capacity ratings for furnaces, air conditioners and heat pumps.

Fuels

Energy content of natural gas, oil and coal.

Appliance specifications

HVAC systems, water heaters and gas stoves in the US.

Kilo British Thermal Unit (kBTU)

One thousand BTUs, about 1.055 megajoules. Building energy performance in the United States is normally reported in kBTU per square foot per year.

Building energy

Energy use intensity benchmarks for commercial buildings.

HVAC systems

Output ratings of furnaces and large air conditioners.

Fuel accounting

Energy content of delivered natural gas and heating oil.

Joule (J)

The SI unit of energy, work and heat: the work done when a force of one newton moves through one metre. It is equally one watt-second, and one coulomb-volt.

Physics

Kinetic, potential and thermal energy in all SI calculations.

Electrical work

Energy delivered by a current through a resistance.

Thermodynamics

Heat transfer and reaction enthalpies.

Kilojoule (KJ)

One thousand joules. Outside the United States, food energy is labelled in kilojoules, often alongside the kilocalorie figure.

Nutrition

Food energy on labels in the EU, UK and Australia.

Chemistry

Reaction enthalpies, in kilojoules per mole.

Engineering

Energy in mechanical work and heating processes.

Megajoule (MJ)

One million joules. A kilowatt-hour is 3.6 MJ, and the energy content of fuels is commonly given in megajoules per kilogram or per litre.

Fuel energy

Calorific value of petrol, diesel, gas and biofuels.

Power generation

Energy flows in plant and process engineering.

Transport

Vehicle energy consumption per kilometre.

Gigajoule (GJ)

One billion joules, or about 278 kilowatt-hours. District heating and industrial gas supply are frequently billed by the gigajoule.

District heating

The billed unit for delivered heat in many countries.

Fuel supply

Natural gas contracts and energy content of coal shipments.

Industry

Energy budgets for manufacturing and mining processes.

Gram calorie (gram-cal)

The heat needed to raise one gram of water by one degree Celsius, defined as exactly 4.184 joules (the thermochemical calorie). This is the small calorie; the one on food labels is a thousand times larger.

Chemistry

Reaction heats and calorimetry in laboratory work.

Physics

Specific heat capacities in older literature.

Kilo calorie ([k]cal)

One thousand gram calories, or exactly 4 184 joules. This is the calorie of nutrition: when a food label says 200 calories, it means 200 kilocalories.

Nutrition

Food energy on labels and in dietary guidance.

Exercise

Energy expenditure reported by fitness trackers.

Calorimetry

Heat measurement in laboratory experiments.

Why so many units for one quantity

Energy is a single physical quantity, but each field developed its own unit before the connection between heat, work, and electricity was understood. Chemists measured heat in calories, engineers measured it in British thermal units, electrical utilities billed in watt-hours, and physicists working with particles needed something far smaller.

All of them measure the same thing, and all convert to joules by a fixed factor. The joule is the SI unit and the natural common ground: one joule is the work done by a force of one newton acting over one meter.

The units in practice

Joule

The SI unit, and the reference every other unit here converts through. A joule is a small amount on a human scale — roughly the energy needed to lift an apple one meter — so practical quantities are usually expressed in kilojoules or megajoules.

Watt-hour and kilowatt-hour

Energy expressed as power sustained over time. One watt for one hour is 3,600 joules, since a watt is one joule per second. The kilowatt-hour is the unit on your electricity bill, and megawatt-hours and gigawatt-hours scale it up for generation and grid figures.

Calorie and kilocalorie

Originally the heat required to raise one gram of water by one degree Celsius. The gram calorie is 4.184 joules. The kilocalorie is a thousand of those — and it is what food labels mean, whether they print "Calories" or "kcal".

British thermal unit

The heat needed to raise one pound of water by one degree Fahrenheit, about 1,055 joules. It remains standard for heating and air conditioning equipment, especially in the United States, where boiler and air conditioner capacity is quoted in BTU or kBTU.

Electronvolt

The energy an electron gains crossing a potential difference of one volt: about 1.602 × 10⁻¹⁹ joules. It is the natural unit for atomic and particle physics, where joules would mean writing impractically small numbers. Chemical bonds are a few eV; nuclear processes run to MeV.

A note on food calories

This causes real confusion. The "calorie" on a nutrition label is a kilocalorie — one thousand of the calories used in physics and chemistry. A 250-calorie snack contains 250 kilocalories, which is about 1,046 kilojoules.

The convention is to capitalize the food unit as "Calorie" and leave the scientific one lowercase, but this is followed inconsistently and is invisible in speech. Labels in the European Union and Australia sidestep the problem by printing kilojoules alongside, which is why the same product can show both 250 kcal and 1,046 kJ.

Worked examples

• A 60 W bulb left on for 10 hours— 600 watt-hours, or 0.6 kWh, or 2.16 megajoules.
• A 2,000 kcal daily diet— about 8,368 kilojoules.
• A 12,000 BTU air conditioner— about 12.66 megajoules per hour, or roughly 3.5 kW of cooling.
• A month of household electricity at 300 kWh— 1.08 gigajoules.
• A typical chemical bond at 4 eV— about 6.4 × 10⁻¹⁹ joules per bond.

Common uses

• Electricity bills— understanding kilowatt-hours and what an appliance actually costs to run.
• Heating and cooling— comparing equipment rated in BTU against equipment rated in kilowatts.
• Nutrition— converting between kilocalories and kilojoules on food labels.
• Physics and chemistry— bond energies, reaction enthalpies, and particle energies.
• Engineering— reconciling datasheets written in different unit conventions.
• Energy reporting— comparing consumption figures published in joules, kWh, or BTU.