Calculate pump shaft power.
Estimate hydraulic power, required shaft power and a user-selected power reserve from the duty point.
Hydraulic power and shaft power
Flow, head and fluid density determine hydraulic power transferred to the liquid. Overall pump efficiency then gives the required mechanical power at the pump shaft.
Phyd = ρ · g · Q · H
Pshaft = Phyd / η
Pwith reserve = f · Pshaft
Internally: ρ in kg/m³, Q in m³/s, H in m and g = 9.80665 m/s². Power is calculated in watts and displayed in kW and mechanical horsepower (hp, not metric PS).
What does this mean for motor selection?
The reserve factor is project-dependent. A generic allowance is not a complete sizing verification. Also consider maximum power demand across the operating range, starting, transmission components, duty, frequency converter, cooling and manufacturer data.
Shaft power is not the motor's electrical input power. The latter also requires motor and, where relevant, converter efficiency. The result with reserve is not yet a selection of a standard motor size.