As we move toward the left side of the curve, the horsepower required to run the pump drops. This may be counter-intuitive to some, but the fact is that most of a pump's energy goes into producing flow, not pressure. Figure 2 is the pump curve for a 3-HP pump and includes a shaft power curve. Notice that, as the flow is reduced, the horsepower required to run the pump drops dramatically. At 90 gpm, it takes 3 ½ HP to run this pump, and at 20 gpm, only about half that amount. This is the order of magnitude of energy savings that can be realized using a throttling valve.