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MasterMath

Electrical Power Calculator

Electrical power is voltage times current. It is what your appliances are rated in and what your meter actually charges for.

Result

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How this was worked out

    The formula

    P = V · I

    SymbolQuantityUnit
    PPowerW
    VVoltageV
    ICurrentA

    What it means

    Power is the rate at which energy is used. Multiplying voltage by current gives watts, and combining it with Ohm's law gives two more useful forms: P = I²R and P = V²/R.

    How to work it out by hand

    1. Multiply the voltage by the current
    2. The answer is in watts
    3. Multiply by hours for the energy in watt-hours

    What is worth knowing

    The I²R form explains why power is transmitted at high voltage: for the same power, higher voltage means lower current, and losses in the cable go with the square of the current. Doubling the transmission voltage cuts the losses to a quarter, which is why grids run at hundreds of kilovolts. Your bill, meanwhile, is in kilowatt-hours, which is energy rather than power: a 2000 W heater running for half an hour uses one kilowatt-hour, exactly the same as a 100 W bulb left on for ten.

    Frequently asked questions

    What is the difference between watts and watt-hours?

    Watts measure power, the rate of energy use. Watt-hours measure energy: watts multiplied by how long it runs.

    Why is electricity transmitted at high voltage?

    Because for the same power, higher voltage means lower current, and cable losses grow with the square of the current.

    How do I work out an appliance's running cost?

    Multiply its watts by the hours used, divide by 1000 for kilowatt-hours, then multiply by your tariff.

    What are the other forms of the formula?

    P = I²R and P = V²/R, both from substituting Ohm's law. Use whichever matches the values you have.