Tony M . . . The electric fan takes energy to turn and that energy comes from the alternator which in turn gets its energy from the crank shaft.

The electric fan blades are no doubt more efficient than the metal blades on the front of the water pump. Overall, the effective power available to the rear wheels on an intermittant basis is increased when the fan is run from the battery/alternator - an electric fan requires the same energy at any speed - the mechanical fan requires more energy the faster you spin it. Running the fan on electricity provides a load leveling effect that is not affected by motor speed.