Energy and Work Unit Converter

Convert between energy and work units including joules, kilowatt-hours, calories, BTU, electronvolts, and more.

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Understanding Energy and Work

What is a Joule?

The joule (J) is the SI unit of energy and work. One joule is defined as the amount of energy transferred when one newton of force moves an object one meter.

1 J = 1 N·m = 1 kg·m²/s² = 1 W·s

Energy = Power × Time

Energy and power are related by time. If you know the power consumption (watts) and duration (seconds or hours), you can calculate energy.

E = P × t

Example: A 100-watt light bulb running for 10 hours uses 100 W × 10 h = 1,000 Wh = 1 kWh of energy.

Kilowatt-hours (kWh)

The kilowatt-hour is the standard unit for electricity billing. One kWh equals 3.6 megajoules (3,600,000 J). A typical US household uses about 30 kWh per day, or 900 kWh per month.

Calorie vs calorie: A Critical Distinction

The word "calorie" has two different meanings depending on capitalization, which creates confusion in nutrition and science:

Small calorie (cal)

Also called: gram calorie, thermochemical calorie

Definition: Energy to raise 1 gram of water by 1°C

Value: 1 cal = 4.184 J

Large Calorie (Cal)

Also called: kilocalorie (kcal), food Calorie

Definition: Energy to raise 1 kilogram of water by 1°C

Value: 1 Cal = 1 kcal = 1,000 cal = 4,184 J

Important: All nutrition labels use the large Calorie (Cal). When a food package says "240 Calories," it means 240 kcal = 240,000 cal = 1,004,160 J (about 1 MJ).

British Thermal Unit (BTU)

The BTU is commonly used in HVAC and heating systems in the United States. One BTU is approximately 1,055 joules. Air conditioner capacity is rated in BTU/hr (e.g., 12,000 BTU/hr = 1 ton of cooling).

TNT Equivalent

The ton of TNT is a unit used to describe the energy released in explosions. It's defined as exactly 4.184 gigajoules (1 billion calories). Nuclear weapons are measured in kilotons (thousands of tons) or megatons (millions of tons) of TNT equivalent.

Electronvolt (eV)

The electronvolt is the standard energy unit in atomic and particle physics. One eV is the kinetic energy gained by an electron accelerated through a potential difference of one volt.

1 eV = 1.602176634 × 10⁻¹⁹ J

X-rays: keV range • Gamma rays: MeV range • Particle accelerators: GeV-TeV range

Energy Conservation

The First Law of Thermodynamics states that energy cannot be created or destroyed, only converted from one form to another. When you lift an object, chemical energy from your muscles converts to gravitational potential energy. When it falls, that converts to kinetic energy, then to heat and sound upon impact.

Energy Density Comparison

Fuel/Source	Energy Density	Notes
Hydrogen (compressed)	142 MJ/kg	Highest chemical fuel by mass
Gasoline	46 MJ/kg	32 MJ/L by volume
Lithium-ion battery	0.4-0.9 MJ/kg	Rechargeable, improving
TNT	4.6 MJ/kg	Explosive chemical energy
Uranium-235 (fission)	88,000,000 MJ/kg	Nuclear energy, million times greater

Common Fuel Energy Content

Gasoline: ≈ 46 MJ/kg or ≈ 120 MJ/gallon
Diesel: ≈ 45 MJ/kg or ≈ 130 MJ/gallon (denser than gasoline)
Natural gas: ≈ 55 MJ/kg or ≈ 37 MJ/m³
Coal (bituminous): ≈ 24-35 MJ/kg
Wood (dry): ≈ 16 MJ/kg

Human Energy Expenditure

Basal metabolic rate: ≈ 1,400-1,800 Cal/day (5.9-7.5 MJ/day)
Walking (moderate pace): ≈ 280 Cal/hour (1.17 MJ/hour)
Running (6 mph): ≈ 600 Cal/hour (2.51 MJ/hour)
Cycling (moderate): ≈ 400 Cal/hour (1.67 MJ/hour)

Most Common Conversions

Conversion	Example Result
Gigajoules to Joules (GJ to J)	1 GJ = 1,000,000,000 J
Joules to Gigajoules (J to GJ)	1,000,000,000 J = 1 GJ
Joules to Kilowatt-hours (J to kWh)	3,600,000 J = 1 kWh
Kilowatt-hours to Joules (kWh to J)	1 kWh = 3,600,000 J
Kilowatt-hours to Gigajoules (kWh to GJ)	1 kWh = 0.0036 GJ
Gigajoules to Kilowatt-hours (GJ to kWh)	1 GJ = 277.778 kWh
Megajoules to Nutrition Calories (MJ to Cal)	1 MJ = 239.006 Cal
Nutrition Calories to Megajoules (Cal to MJ)	1 Cal = 0.004184 MJ

Quick Reference Cards

Electrical Energy

1 kWh = 3.6 MJ

1 kWh = 3,412 BTU

1 kWh = 860 kcal

Standard electricity billing unit

Food Energy

1 Cal = 1 kcal = 4.184 kJ

1 Cal = 1,000 cal

2,000 Cal/day = 8.4 MJ/day

Nutrition labeling (capital C)

Heating/Cooling

1 BTU = 1,055 J

1 therm = 100,000 BTU

1 ton cooling = 12,000 BTU/hr

HVAC industry standard

Mechanical Work

1 ft·lbf = 1.356 J

1 hp·h = 2.685 MJ

1 J = 10⁷ erg

Mechanical engineering

Energy Examples

AA Battery

≈ 9,360 J

≈ 2.6 Wh

Typical alkaline

Chocolate Bar

≈ 1,000 kJ

≈ 240 Cal

Food energy

Gallon of Gasoline

≈ 120 MJ

≈ 33 kWh

Chemical energy

Ton of TNT

≈ 4.184 GJ

≈ 1,162 kWh

Explosive energy

Lightning Bolt

≈ 1-5 GJ

≈ 250 kWh

Typical strike

Hiroshima Bomb

≈ 15 kilotons

≈ 63 TJ

Historical (1945)

Sun's Annual Output

≈ 1.2 × 10³⁴ J

≈ 3.3 × 10²⁷ kWh

One year of solar radiation

Observable Universe

≈ 4 × 10⁶⁹ J

≈ 10⁶³ kWh

Total mass-energy content

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