Ergs to Watt-hours (erg to Wh)

1 erg equals 2.777777778×10⁻¹¹ watt-hours (rounded).

1 erg = 2.777777778×10⁻¹¹ watt-hours

How to convert ergs to watt-hours

To convert ergs to watt-hours, multiply the number of ergs by 2.777777778×10⁻¹¹. The factor is rounded to 10 significant digits. To go the other way, multiply watt-hours by 36,000,000,000.

watt-hours = ergs × 2.777777778×10⁻¹¹

Example: 10 ergs = 2.777777778×10⁻¹⁰ 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

Erg (erg)

CGS & old metricTraditional unit (still used in some regions)

Exactly 10⁻⁷ joules. The energy unit of the old centimeter-gram-second (CGS) system, still used in astrophysics.

Erg takes its name from the Greek ergon, meaning work. Rudolf Clausius proposed that word for a unit of energy in 1864, and in 1873 a British Association committee including Maxwell and Thomson asked that erg be reserved for the energy unit of the centimeter-gram-second system.

It is the work done by a force of one dyne acting through one centimeter, or 1 g cm2/s2. That equals 10 to the power minus 7 joule, 100 nanojoules, which is a very small amount of energy and was a natural size for laboratory experiments of the era.

The erg is not an SI unit, and NIST lists it only as a conversion factor to the joule. It survives mainly in astrophysics and theoretical physics, where Gaussian CGS units remain common, so cosmic energies are still often given in ergs.

1 erg = 1×10⁻⁷ joules (exactly).

Sources: Wikipedia: Erg (unit); Wikipedia: Centimetre-gram-second system of units; NIST SP 811, Appendix B.8 conversion factors

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.