The selection of a suitable size of prime mover for an a.c. generator is governed by the electrical power output supplied to the load and the efficiency of a.c. generator. The relationship is given below :

                        kW outputkW
kW drive input = _______________________ 
                 Efficiency of alternator

"kW drive input" defines the prime mover rating in kilowatts

"kW output" defines the electrical power supplied to load (usually the continuous maximum industrial power rating, of a.c. generator).

To convert the kW rating into horse power the following conversion is used.

        kW input
H.P. = _________ 


To determine the minimum size rating of the engine to drive an AC Generator for 125 kVA, 4 pole, 1500 rpm at 0.8 pf with an efficiency of 92%

kW = 125 X 0.8 kW

                  100 kW
kW drive input = ________

               = 109 kW 
                  kW drive input
In terms of HP = ________________ 0.746 

               = 146 HP