The apparent power is the vector sum of real and reactive power.

Engineers use the following terms to describe energy flow in a system (and assign each of them a different unit to differentiate between them):

• Real power (P) [Unit: W]
• Reactive power (Q) [Unit: VAR]
• Complex power (S)
• Apparent Power (|S|) [Unit: VA]: i.e. the absolute value of complex power S.

P is the real power, Q is the reactive power (in this case negative), S is the complex power and the length of S is the apparent power.

The unit for all forms of power is the watt (symbol: W). However, this unit is generally reserved for the real power component. Apparent power is conventionally expressed in volt-amperes (VA) since it is the simple product of rms voltage and rms current. The unit for reactive power is given the special name "VAR", which stands for volt-amperes reactive (since reactive power flow transfers no net energy to the load, it is sometimes called "wattless" power). Note that it does not make sense to assign a single unit to complex power because it is a complex number and it is therefore defined as a pair of two units: W and VAR.