Millijoules to Watt-hours (mJ to Wh)

1 millijoule equals 2.777777778×10⁻⁷ watt-hours (rounded).

1 millijoule = 2.777777778×10⁻⁷ watt-hours

How to convert millijoules to watt-hours

To convert millijoules to watt-hours, multiply the number of millijoules by 2.777777778×10⁻⁷. The factor is rounded to 10 significant digits. To go the other way, multiply watt-hours by 3,600,000.

watt-hours = millijoules × 2.777777778×10⁻⁷

Example: 10 millijoules = 0.000002777777778 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

Millijoule (mJ)

Metric (SI)Current unit

One thousandth of a joule. Used for very small amounts of energy.

Prefixing the joule with milli gives a unit one thousandth of the base size, so 1 mJ is 0.001 J. The joule itself honours the English physicist James Prescott Joule, and its name was proposed by Wilhelm Siemens in 1882 before adoption at the 1889 International Electrical Congress. Because the joule equals one watt-second, a millijoule is also a milliwatt applied for one second, which makes it a natural size for tiny electrical budgets.

Some verifiable comparisons show the scale. One list of energy magnitudes puts an atomic battery's output over an hour at about 3 mJ, and an LED running for a second at about 40 mJ. A half-dollar coin dropped from a metre releases roughly 110 mJ. In atomic units, one millijoule is about 6.24 x 10^15 electronvolts, using the exact electronvolt value of 1.602176634 x 10^-19 J. Below the millijoule come the microjoule and nanojoule, so it sits at the coarse end of small-energy measurement.

1 millijoule = 0.001 joules (exactly).

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

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.