How to Calculate Electric Energy in kWh and Its Cost

Calculates electric energy as power × time. Power is in watts, time in hours, and energy in watt-hours; 1000 Wh is 1 kWh. Multiplying by the unit price gives the cost.

Explanation

Electric energy is the total amount of electricity used, as opposed to the rate at which it is being used. Multiply the power by the hours it ran, then multiply by your unit price to get the cost.

E = P t

$E$ is the energy, reported here in kilowatt-hours (kWh)
$P$ is the power, in watts (W)
$t$ is the running time, in hours (h)

Watts times hours gives watt-hours, and 1000 Wh make 1 kWh, so the calculator divides the product by 1000.

Example

The defaults are 600 W running for 3 h, at a unit price of 31 per kWh.

E = \dfrac{600 \times 3}{1000} = 1.8\,\mathrm{kWh}

The energy is 1.8 kWh and the cost is 1.8 × 31 = 55.8. With the Japanese reference rate of 31 yen per kWh, running a 600 W appliance for three hours costs about 56 yen.

Notes

Do not confuse power with energy. Watts are the rate of use at any instant; kilowatt-hours are the amount used over time, and that is what gets billed.

The time goes in hours. Thirty minutes is 0.5 h.

The unit price depends on your contract and, on some tariffs, on the time of day. The default of 31 is the standard reference figure used in Japan, in yen per kWh, so replace it with the rate on your own bill. Standing charges and levies are not included.

One kilowatt-hour is 3.6 MJ. It is the same kind of energy a joule measures, just written at a size that suits a household.