An electric heater works by resistance: the heating element has high resistance to the electricity passing through it. That resistance, reckoned in ohms, generates the heat you feel rising from the heater. If you want to know how many watts of power a heater uses, you must understand the relationship between ohms, volts and amps and how volts and amps result in wattage.

Look at the plate on the heater that tells how many volts the heater requires to operate and the resistance, shown in ohms, of the heating element. Record both numbers.

Divide the number of volts by the number of ohms to determine the number of amperes, or amps, using Ohm's Law: E/r=I, where E is the voltage, I is the number of amps and r is the resistance of the circuit in ohms.

Multiply the number of volts by the number of amps to determine the wattage of the heater, using Watt's Law: E x I=P, where E is the voltage, I is the number of amps and P is the power, expressed in watts.
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