How to Calculate Watts

Watch more Intermediate Math Skills videos: howcast.com/videos/437615-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.

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