You Will Need
 Appliance
 Mathematical formulas
Steps

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.