Pump Power and Head Calculator

How is pump power calculated?

The useful power a pump gives to the fluid is the hydraulic power: P_h = ρ × g × Q × H, where ρ is density, g is gravity, Q is flow rate and H is head. The same power equals flow times pressure rise: P_h = Q × Δp.

Because of losses inside the pump, the shaft needs more: shaft power = hydraulic power / pump efficiency. The motor in turn draws more from the supply by its own losses: electrical power = shaft power / motor efficiency.

How head relates to pressure

Δp = ρ × g × H. For water, 10 metres of head is about 0.98 bar. Head is expressed in metres of the pumped fluid, so the same pump delivers the same head on a denser fluid but produces more pressure and draws more power.

Example

A pump moving 50 cubic metres of water per hour against 30 metres of head has a hydraulic power of 1000 × 9.80665 × (50 / 3600) × 30 = 4086 W. At 70 percent pump efficiency the shaft power is 5.84 kW, and at 90 percent motor efficiency the electrical input is 6.49 kW. A 7.5 kW motor suits this pump.

Limits

The calculator gives power at the duty point. Power varies along the pump curve, so the motor is sized for the highest power point. Suction side cavitation checks (NPSH) and pipe losses are outside this tool.