The efficiency of a dynamo is the quotient obtained by dividing the amount of electric power furnished by the dynamo by the amount of mechanical power delivered to the dynamo. It is measured by indicating the engine while running the dynamo at full load and noting the reading of the ammeter and voltmeter, and then indicating the engine when the dynamo is idle. The difference between the two readings is approximately the mechanical power supplied to the dynamo.

Watts / 746 = Volts X Amperes / 746 = Horse-Power

Motors are rated in horse-power (H.P.) Dynamos are rated in kilowatts (kw.).

1H.P. = 3/4 kw.