Notice also that as you throttle the flow through the pump, the horsepower drops. Some people instinctively think that if you throttle the pump it must take more power, but this is not so. Check the power curve in Figure 1. The right end of the power curve may drop slightly since, at the very low head generated, very little work is being done. You should not be operating your pump in this unstable region.
Manufacturers often give the power taken at maximum flow, in watts.
If you remember that there are 746 watts to one horsepower, you will often find that the maximum power drawn is well above the horsepower rating of the motor. A good rule of thumb would be to operate the pump at not more than two thirds of maximum flow. This is the region where the efficiency is at a maximum and the power drawn per gallon pumped is at a minimum. Don't make the mistake of buying a cheaper pump which is really too small for the flow required, and then running it at near maximum flow. Its life will be greatly reduced.