Pump Power Calculator
Calculate the shaft power and motor input power needed to pump a fluid against a given head. Use this for selecting pump and motor sizes in water supply, irrigation, and industrial systems.
About this calculator
The hydraulic power delivered to a fluid by a pump is P_hydraulic = ρ × g × Q × H, where ρ is fluid density (kg/m³), g = 9.81 m/s², Q is volumetric flow rate (m³/s), and H is the total dynamic head (m). Because no pump or motor is perfectly efficient, the actual electrical input power is higher: P_input (kW) = (ρ × g × Q × H) / (η_pump × η_motor × 1000). The pump efficiency η_pump accounts for hydraulic, volumetric, and mechanical losses within the pump itself, while motor efficiency η_motor covers electrical losses in the drive motor. Total dynamic head H includes static lift, friction losses in pipes, and minor losses through valves and fittings. Accurate sizing prevents undersized pumps from failing to meet demand and oversized pumps from wasting energy.
How to use
You need to pump water (ρ = 1000 kg/m³) at Q = 0.02 m³/s against a total head of H = 30 m. Your pump has 75% efficiency (η_pump = 0.75) and the motor is 90% efficient (η_motor = 0.90). Input power = (1000 × 9.81 × 0.02 × 30) / (0.75 × 0.90 × 1000) = 5,886 / 675 ≈ 8.72 kW. This means you need a motor rated at least 8.72 kW to operate this system. Enter your flow rate, head, fluid density, and efficiencies above to size your pump motor instantly.
Frequently asked questions
What is total dynamic head and how do I calculate it for a pump system?
Total dynamic head (TDH) is the equivalent height of fluid that the pump must raise, expressed in metres. It includes the static head (actual elevation difference between inlet and outlet), the friction head (pressure losses due to pipe friction calculated via Darcy-Weisbach), and minor losses (valves, bends, and fittings expressed as equivalent lengths). Velocity head differences between suction and discharge are usually small but can be included for precision. TDH is calculated by summing all these components: TDH = static head + friction head + minor losses. It is the key input for selecting the correct pump from a manufacturer's performance curve.
How does pump efficiency affect electricity costs over time?
Pump efficiency directly determines how much electrical energy is wasted as heat for every unit of useful hydraulic work delivered. A pump operating at 60% efficiency consumes 67% more electricity than an ideal 100%-efficient machine for the same output. Over a year of continuous operation, even a 5-percentage-point improvement in efficiency can save thousands of kilowatt-hours and hundreds of dollars in energy costs. This is why high-efficiency pumps with better impeller designs often justify their higher capital cost through rapid payback. Always consider lifecycle energy costs, not just purchase price, when selecting pumps.
What is the difference between pump efficiency and motor efficiency?
Pump efficiency (η_pump) describes how well the pump converts shaft mechanical power into useful hydraulic power delivered to the fluid, accounting for internal hydraulic, volumetric, and bearing losses. Motor efficiency (η_motor) describes how well the electric motor converts electrical input power into shaft mechanical power, accounting for copper losses, iron losses, and friction in the motor. The overall system efficiency is the product of both: η_total = η_pump × η_motor. To minimize energy use, both components should be selected at their best efficiency point (BEP), and variable-frequency drives (VFDs) can further improve part-load performance.