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How to Calculate Watts

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


  • 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.
  • TIP: Equivalently 1 watt is the amount of work done at the rate of one joule per second.
  • 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).
  • TIP: 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).
  • 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.
  • FACT: James Watt was an 18th-century mechanical engineer who made important contributions to improvements of the steam engine.

You Will Need

  • Appliance
  • Mathematical formulas

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