Calories (IT) to Watt-hours (cal (IT) to Wh)

1 calorie (IT) equals 0.001163 watt-hours exactly.

1 calorie (IT) = 0.001163 watt-hours

How to convert calories (IT) to watt-hours

To convert calories (IT) to watt-hours, multiply the number of calories (IT) by 0.001163. This factor is exact. To go the other way, multiply watt-hours by 859.8452279 (rounded).

watt-hours = calories (IT) × 0.001163

Example: 10 calories (IT) = 0.01163 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

Calorie (IT) (cal (IT))

ScientificHistorical unit (no longer in standard use)

The International Table calorie: exactly 4.1868 joules, slightly larger than the thermochemical calorie. Used in steam tables.

Steam engineers needed a calorie tied to the joule rather than to the behaviour of water, and the Fifth International Conference on the Properties of Steam, held in London in July 1956, fixed one. The result, the International Table calorie, is exactly 4.1868 J, defined as 1.163 milliwatt-hours. An earlier International Steam Table calorie of 1929 was about 4.1860 J, so the 1956 choice slightly adjusted the older figure.

Compared with the thermochemical calorie of 4.184 J, the International Table version is larger by 0.0028 J, or roughly 0.07 percent. That is negligible for cooking but can matter in steam tables and engineering handbooks, where the calorie and its kilocalorie multiple of 4,186.8 J are used. NIST lists both calories as exact values and flags them separately for that reason. The figure used here for this unit is 4.1868 J. Anyone converting data should check which calorie a source assumes before comparing numbers.

1 calorie (IT) = 4.1868 joules (exactly).

Sources: Wikipedia: Calorie; NIST SP 811, Appendix B.8 (energy conversion factors); Wikipedia: Conversion of units of energy

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.