Last updated: 2026-05-14

Energy

Energy is the capacity to do work. The SI unit is the joule (J), defined as the work done when a force of one newton moves an object one metre. In practice, many different units are used across food science, electricity, engineering, and particle physics.

Units covered

Joules (J) — the SI base unit. One joule is the kinetic energy of a 2 kg mass moving at 1 m/s. A 100-watt light bulb consumes 100 joules per second.

Kilojoules (kJ) — 1,000 joules. Commonly used in nutrition and chemistry. Food labels in many countries report energy in kJ alongside kcal.

Kilocalories (kcal) — the "food calorie." One kcal equals 4,184 J. When a nutrition label says a food contains "300 calories," it means 300 kcal. The thermochemical calorie (4.184 J for 1 cal) is the basis of this conversion.

Watt-hours (Wh) — the energy consumed by a one-watt device running for one hour. Equals 3,600 J. Used for battery capacity ratings (a typical AA battery holds about 3–4 Wh).

Kilowatt-hours (kWh) — 1,000 Wh = 3,600,000 J. The standard unit on electricity bills. The average US household uses about 30 kWh per day.

British Thermal Units (BTU) — the heat needed to raise one pound of water by 1°F. Equals 1,055.06 J (ISO definition). Used in HVAC (air conditioning capacity, furnace output) and US energy statistics.

Electronvolts (eV) — the energy gained by one electron accelerated through a potential of one volt. Equal to 1.602 × 10⁻¹⁹ J. Used in atomic physics, chemistry (bond energies), and semiconductor electronics.

Key conversion table

From 1 unit	Joules
1 kJ	1,000 J
1 kcal	4,184 J
1 Wh	3,600 J
1 kWh	3,600,000 J
1 BTU	1,055.06 J
1 eV	1.602 × 10⁻¹⁹ J

Usage

Enter any energy value in any field. All other units update automatically. For example, entering 2000 in the kcal field immediately shows that 2,000 kcal (a typical daily calorie intake) equals 8,368 kJ, 2.324 kWh, or 7,933 BTU.

Frequently Asked Questions (FAQ)

What is the difference between a joule and a calorie?

A joule (J) is the SI unit of energy: the work done when a force of one newton moves an object one metre. A calorie (cal) is a unit from chemistry: the energy needed to raise one gram of water by 1°C at standard pressure, equal to 4.184 J. Food labels use the kilocalorie (kcal) — also called the "large calorie" or "food calorie" — which equals 4,184 J. When a food label says "200 calories" it means 200 kcal.

When should I use kWh vs kJ?

Both measure the same thing. Kilowatt-hours (kWh) are the standard unit for household electricity: 1 kWh = 3,600 kJ = 3,600,000 J. Kilojoules (kJ) appear in physics, chemistry, and nutrition contexts. Your electricity bill uses kWh because it measures energy delivered over time (1 kilowatt consumed for 1 hour). Engineers often prefer kJ for thermodynamic calculations.

What is an electronvolt and when is it used?

An electronvolt (eV) is the kinetic energy gained by one electron accelerated through a potential difference of one volt. It equals 1.602 × 10⁻¹⁹ J — an extremely small unit.

Electronvolts are used in atomic physics, particle physics, and solid-state electronics where energies at the molecular or subatomic scale are involved. For comparison, a chemical bond has an energy of a few eV; a proton in the Large Hadron Collider carries trillions of eV (TeV).

When would I use BTU?

BTU appears most often in the US when sizing HVAC equipment. An air conditioner's cooling capacity is rated in BTU/hour (often called just "BTU"), and furnaces are rated in BTU/hour of heat output. A 12,000 BTU/h air conditioner can remove 12,000 BTU of heat per hour — about 3,516 watts of cooling capacity.