Power is a physics term used to describe the average amount of energy that is transferred per unit of time. Thus power is expressed in units that are units of energy divided by units of time, the most commonly used of these units being Watts. Measuring electrical power is a matter of measuring its individual components and then performing a relatively simple calculation.
Things You'll Need
 Electrical multimeter

Use the multimeter to measure the voltage in the circuit and record the value in Volts. We will call this value "V."

Use the multimeter to then measure the current in the circuit and record the value in Amperes. We will call this value "A."

Substitute the values for "V" and "A" into the equation to calculate power in a circuit using voltage and current and calculate.
P (power) = V*A 
For example, in a circuit with a voltage of 120 Volts and a current of 20 Amperes, the power is equal to 120 times 20, or 2400 VoltAmperes.

Convert the VoltAmperes to Watts to properly express the result in power units. Expressing your voltage and current in Volts and Amperes respectively makes this an easy job because 1 VoltAmpere = 1 Watt. So for the above example, the power is 2400 Watts.