Apparent power (S) is expressed as the product of RMS voltage and RMS current. The apparent power is used during designing of transmission equipment. In terms of value, it is equal to maximum active power which can be supplied to a load at given RMS voltage and current. Thus, the apparent power defines the maximum capacity of the source to supply usable energy to the receiver.
The measure of effective use of supplied power by the receiver is the power factor, which is the ratio of active power to apparent power.