How to Calculate Watts

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

You will need

  • Appliance
  • Mathematical formulas

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.

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.

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.

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).

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.

Step 6 Use voltage and resistance Calculate the wattage from the quotient of the square of voltage divided by resistance.