Power is an expression of an amount of work, or energy transferred, in a unit of time. Power itself is not directly measured, but instead can be calculated using a number of formulas that use other values that are directly measured. The result of these calculations is expressed in energy units divided by time units, the international standard unit of power being the Watt. Horsepower is often used to express power in the U.S.

Record the value for the current in Amperes, and call this value "A."

Record the value for the voltage in Volts, and call this value "V."

Record the value for the efficiency as a decimal and call this value "E."

Substitute the values into the following equation that yields power as its result.
P = V A E 
For example, for a motor that operates at 85% efficiency, and runs at 220 volts with a current of 30 amperes, the equation would be as follows:
P = 22030.85 = 5610 Watts