For a transformer, apparent power output is calculated as S = V? × I? for single phase, or S = ?3 × V? × I? for three phase, where V? is the secondary line voltage and I? is the line current. This gives kVA or MVA. Real power output is P = S × power factor. When calculating, engineers use rated voltages and currents from the nameplate for maximum output, or measured load values for real operating power. They also consider efficiency, since some input power is lost as heat and not delivered to the load.