How to Calculate Watts

The amount of power drawn by a household appliance is given in watts. Here's how to calculate wattage.


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You Will Need

  • Appliance
  • Mathematical formulas


  1. Step 1

    Understand the definition

    Know that 1 watt is the amount of power produced by a current of 1 ampere across a potential difference of 1 volt.

  2. Equivalently 1 watt is the amount of work done at the rate of one joule per second.

  3. Step 2

    Understand appliances

    Understand that the wattage listed on an appliance is the maximum power it draws. Because many appliances have a range of settings, the actual amount of power consumed at any given time depends on the setting used.

  4. Step 3

    Use a formula

    Consider using a formula to estimate wattage. To do this you will need to know the current draw, the voltage used by the appliance, and/or the resistance of the appliance.

  5. Step 4

    Use voltage and current

    Calculate the wattage from the product of voltage and the current for the appliance. Voltage is measured in volts and current in amperes (or amps).

  6. If an appliance is rated at 5 amps and 120 volts, the wattage is 600 watts (5 amps times 120 volts, which equals 600 watts).

  7. Step 5

    Use current and resistance

    Obtain the wattage as the product of the square of current times the resistance. Resistance is measured in ohms.

  8. Step 6

    Use voltage and resistance

    Calculate the wattage from the quotient of the square of voltage divided by resistance.

  9. James Watt was an 18th-century mechanical engineer who made important contributions to improvements of the steam engine.