Calculation of the apparent power (kVA) supplied by an alternator, knowing the mechanical power available on the engine shaft and the alternator efficiency.

Convert the mechanical power available on the engine shaft in kW.

This means that, if the mechanical power is expressed in CV, this value has to be multiplied by a coefficient equal to 0.735, instead, if the mechanical power is expressed in HP, this value has to be multiplied by a coefficient equal to 0.746.

At this point multiply the mechanical power available on the engine shaft, expressed in kW, by the alternator efficiency. It is then achieved the active power supplied by the alternator, expressed in kW.

As for the 3-phase machines at cosφ = 0.8, the apparent power is achieved by dividing the active power calculated by the coefficient 0.8. As for the 1-phase machines at cosφ = 1, the apparent power is same as the active power calculated.

Of course the apparent power calculated has to be lower or equal to the alternator nominal apparent power.

It is always advisable to select an alternator a little bit oversized against the engine used.

Example: engine with mechanical power available on the shaft of 140 HP and 3-phase alternator rated 130 kVA with 93% efficiency.

The mechanical power available on the engine shaft, expressed in kW, is equal to 104 kW (= 140HP x 0.746). The active power supplied by the alternator is then equal to 97 kW (= 104kW x 0.93). The apparent power supplied by the 3-phase alternator at cosφ = 0.8 is consequently equal to 121 kVA (= 97kW / 0.8).