Calculation Methods of Water Pump Power
2026-06-08
Definition of Water Pump Power
Power refers to the work done by a water pump per unit time, denoted by the symbol N. Its common units include kilogram-meter per second, kilowatt and horsepower. The power of electric motors is measured in kilowatts, while that of diesel or gasoline engines is measured in horsepower.
Definition of Water Pump Effective Power
Effective power, also known as output power, is represented by Pe. It refers to the effective energy obtained by the liquid medium delivered by the pump per unit time.
Head means the energy gained by per unit weight of liquid delivered by the pump. Therefore, the effective energy acquired by the liquid per unit time is exactly the effective power of the pump.
The formula is:
Where: — Density of the delivered liquid, — Specific weight of the delivered liquid, — Pump flow rate, — Pump head, — Gravitational acceleration,
The power loss inside the pump equals the difference between shaft power P and effective power Pe. Pump efficiency, denoted by , is the ratio of effective power to shaft power, which is used to evaluate power loss.
The work done by the pump increases with larger liquid delivery volume and decreases with smaller output, accompanied by reduced electric current.
The calculating formula for pump power (kW):
Motor power is calculated by the formula , so higher power corresponds to larger electric current.
Power loss is inevitable due to friction of bearings and packing, friction between rotating impellers and water, eddy current, clearance backflow and impact at inlet and outlet inside the pump. The input power from the driver cannot be fully converted into effective power. Hence:
Calculation Formulas for Water Pump Power
Formula for centrifugal pump power:
Units: Flow rate in cubic meters per hour, Head in meters.
The simplified formula:
Where: — Head, — Flow rate, — Pump efficiency — Shaft power,
The equivalent formula:
where , .
where , .
Unit derivation:
Given
Given
The above derivation explains the unit conversion. The formula calculates hydraulic power, and shaft power is obtained by dividing hydraulic power by pump efficiency.
Let stand for shaft power, for motor power, and for the coefficient (reciprocal of overall efficiency):
The value of varies with shaft power:
- , when
- , when
- , when
Conclusions drawn from formula derivation:
- The actual effective power is fixed for all manufacturers under determined flow rate and head.
- Power consumption depends on pump shaft power rather than rated motor power. For two pumps equipped with 30 kW motors, the one with a shaft power of 20.56 kW is more energy-efficient than the one of 25.18 kW. Pump efficiency is the core factor determining shaft power.
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