Pump power calculator
Work out the motor power a water pump needs (in kW and HP) from its flow, total head and efficiencies — using the standard P = ρgQH relation.
Enter flow in L/s or m³/hr and read off the hydraulic, shaft and motor input power, with an optional run-hours and tariff input to estimate daily energy and running cost in Rs. A planning-level tool, computed in your browser.
Pump duty point
Volume the pump must deliver per second (or per hour).
Static lift plus friction losses across the system.
Hydraulic efficiency of the pump — usually 60–80%.
Electric-motor efficiency — typically 85–95%.
Enter to estimate daily energy use.
Enter a rate to estimate running cost.
Motor input power
23.36 kW
≈ 31.3 HP electrical input at the motor
Shaft power
21.02 kW
Hydraulic power
14.72 kW
Shaft power
21.02 kW
Motor power
23.36 kW
As HP
31.3 HP
| Hydraulic power | P = ρ · g · Q · H |
| Flow (Q) | 30.00 L/s = 0.0300 m³/s |
| Hydraulic kW | 1000 × 9.81 × 0.0300 × 50 ÷ 1000 = 14.72 kW |
| Shaft kW | 14.72 ÷ 0.70 = 21.02 kW |
| Motor kW | 21.02 ÷ 0.90 = 23.36 kW |
| Motor HP | 23.36 × 1.341 = 31.3 HP |
A planning-level estimate of electrical input power. Real selection accounts for the pump and system curves, NPSH, the duty point on the impeller, variable-speed drives, start-up surge and a motor service factor — so the installed motor is usually sized one frame above this figure. Energy and cost figures assume steady run hours and a flat tariff, and are indicative only.
From water power to the meter
The power a pump needs grows with how much water it moves (flow) and how high it lifts it (head); efficiencies then step that figure up from the water to the shaft to the electricity meter.
Hydraulic power
P = ρ·g·Q·H. Convert flow to m³/s, multiply by 1000 kg/m³, 9.81 m/s² and the head, then divide by 1000 for kW — the useful power given to the water.
Shaft power
Divide hydraulic power by the pump efficiency (typically 60–80%) to get the power the pump absorbs at its shaft.
Motor power
Divide shaft power by the motor efficiency (typically 85–95%) for the electrical input in kW, then × 1.341 for HP — plus daily energy and cost.
Pump power, answered
How do you calculate the power of a pump in kW?+
Start with the hydraulic (water) power: P = ρ · g · Q · H, where ρ is water density (1000 kg/m³), g is gravity (9.81 m/s²), Q is the flow in m³/s and H is the total head in metres. Divide by 1000 to get kilowatts. Then divide by the pump efficiency to get shaft power, and by the motor efficiency to get the electrical input power in kW.
What is the difference between hydraulic, shaft and motor power?+
Hydraulic power is the useful power delivered to the water. Shaft power is what the pump actually absorbs at its shaft — hydraulic power divided by the pump efficiency. Motor input power is the electricity the motor draws from the grid — shaft power divided by the motor efficiency. The motor input power is what your bill is based on.
How do I convert flow from m³/hr to L/s?+
Multiply m³/hr by 1000 to get litres per hour, then divide by 3600 to get litres per second (1 m³/hr ≈ 0.2778 L/s). This calculator does the conversion automatically when you switch the flow unit, and converts to m³/s internally for the power formula.
How do I convert pump kW to HP?+
Multiply kilowatts by 1.341 to get horsepower (1 kW ≈ 1.341 HP), or divide HP by 1.341 to go back to kW. This tool shows the motor input power in both kW and HP so you can match it to motor nameplate ratings.
What pump and motor efficiency should I use?+
Centrifugal pump efficiency is typically 60–80% depending on size and duty point — 70% is a reasonable default for a mid-size unit. Electric-motor efficiency is usually 85–95%, so 90% is a sound default. Larger, well-matched pumps and motors sit at the higher end of each range.
Why is the installed motor larger than the calculated power?+
The calculated figure is the power at the duty point. In practice engineers add a margin (a service factor) and pick the next standard motor frame up, to cover start-up surge, off-design operation, wear and voltage dips. Treat this calculator's number as the minimum required, then size up to the nearest available rating.
Sources & data note
Based on the standard pump relation: hydraulic power P = ρgQH, shaft power = hydraulic ÷ pump efficiency, motor input = shaft ÷ motor efficiency, and 1 kW ≈ 1.341 HP. Efficiency defaults (pump 70%, motor 90%) are typical for mid-size centrifugal pumps — confirm against the actual pump and motor curves. Running cost depends on your real electricity tariff; verify the current rate with the NEA. This is a planning estimate, not a substitute for detailed pump selection.