The useful power is that which the engine uses strictly for mechanical work. In other words, the power that is used for the useful work.
On the other hand, the power that comes from the network and is absorbed by the engine, will always be something higher and is related to the useful power depending on the performance of the equipment. It is expressed with this formulation:
Efficiency = Useful power / Power absorbed
This relation must be indicated on the plate of the equipment, so that we know the power and intensity that the group will need from the network.