Gigajoules to Watt-hours (GJ to Wh)

1 gigajoule equals 277,777.7778 watt-hours (rounded).

1 gigajoule = 277,777.7778 watt-hours

How to convert gigajoules to watt-hours

To convert gigajoules to watt-hours, multiply the number of gigajoules by 277,777.7778. The factor is rounded to 10 significant digits. To go the other way, multiply watt-hours by 0.0000036.

watt-hours = gigajoules × 277,777.7778

Example: 10 gigajoules = 2,777,777.778 watt-hours.

The 'calorie' on food labels is really a kilocalorie. The thermochemical calorie (4.184 J) is used here unless marked IT.

About these units

Gigajoule (GJ)

Metric (SI)Current unit

One billion joules, roughly the energy in 29 liters of petrol. Used for natural gas and district heating.

One gigajoule is a billion joules, a size that only shows up when something large is being counted. An average lightning bolt carries about that much, and the superconducting magnet of the ATLAS experiment at CERN stores about 1.1 GJ of magnetic energy.

Everyday life can reach it too, just slowly. Filling a car with about 61 liters of gasoline, roughly 16 US gallons, puts in about 2 GJ of combustion energy. A standard refrigerator uses about 4.5 GJ in a year, and an average human heart muscle expends about 3.3 GJ over an 80-year lifetime.

Larger reference points sit nearby. One megawatt-hour is 3.6 GJ, exploding a ton of TNT releases about 4.2 GJ, and a barrel of oil equivalent is about 6.1 GJ. These comparisons explain why gigajoules suit annual household or building energy tallies better than joules or kilojoules.

1 gigajoule = 1,000,000,000 joules (exactly).

Sources: Wikipedia: Orders of magnitude (energy); Wikipedia: Joule

Watt-hour (Wh)

Metric (SI)Current unit

Exactly 3,600 joules, the energy used by a one-watt device in an hour. Battery capacities are quoted in watt-hours.

Pair a power unit with a unit of time and you get the watt-hour: one watt sustained for one hour, which works out to 3,600 joules. The watt itself was adopted at the second International Electrical Congress in 1889, in the same session that adopted the joule, so the two units share a birthday even though the watt-hour is the one people read off batteries.

Battery capacity is where it earns its keep. Multiplying amp-hours by the battery voltage gives an estimate of watt-hours, so a 2,500 mAh lithium-ion cell at 3.7 V holds roughly 9.25 Wh. That is why a 500 mA USB device runs about 3.7 hours on it rather than five: the cell voltage, not the 5 V from the port, sets the energy.

For scale, an alkaline AA battery holds about 9 kJ, which is 2.5 Wh. A 12 W LED lamp left on for an hour uses 12 Wh, and a 40 W appliance running for 25 hours uses 1,000 Wh.

1 watt-hour = 3,600 joules (exactly).

Sources: Wikipedia: Watt-hour; Wikipedia: Joule; Wikipedia: Orders of magnitude (energy)

Related conversions

See all energy units.