Circuit Theory/Complex Power/apparent power
Practical loads have resistance, inductance, and capacitance, so both real and reactive power will flow to real loads.
Power engineers measure apparent power as the magnitude of the vector sum of real and reactive power.
Apparent power is the product of the root-mean-square of voltage and current.
Engineers working for utility companies care about apparent power, because even though the current associated with reactive power does no work at the load, it heats the wires, wasting energy. Engineers working on antennas (wireless and radar) want the same.
Conductors, transformers and generators must be sized to carry the total current, not just the current that does useful work.
Another consequence is that adding the apparent power for two loads will not accurately give the total apparent power unless they have the same displacement between current and voltage (the same power factor).