Energy consumption is the largest lifetime cost of any pumping system. While purchase price and maintenance are important, inefficiency in pump selection and operation can result in decades of excessive energy bills.
Pump efficiency is the ratio of water horsepower (WHP) delivered to the fluid vs. the brake horsepower (BHP) supplied to the pump shaft.
Efficiency Formula
WHP (kW) = (Q × H × ρ × g) / (3.6 × 106)
Small Pumps
40–70%
Typical efficiency range
Large Pumps
70–90%
Well-designed pumps
Far from BEP
< 40%
May fall below this
Best Efficiency Point (BEP)
Every pump has a flow rate where it performs most efficiently, known as the Best Efficiency Point (BEP). Operating too far left (low flow) or right (high flow) from BEP causes problems.
Effects of Operating Away from BEP
Higher energy consumption
Increased vibration and wear
Premature seal and bearing failures
Key Rule
Always select pumps to operate as close to BEP as possible within the system’s duty range.
Components of Energy Cost
The total energy cost depends on multiple efficiency factors. The overall efficiency is the product of all components.
Pump Efficiency (ηp)
Losses due to internal leakage, friction, and design limitations
Motor Efficiency (ηm)
Losses in the driver motor (typically 90–96% for high-quality motors)
Transmission Efficiency (ηt)
Losses in couplings, gearboxes, or belts
System Efficiency
Includes pipe friction, throttling, and control strategies
Overall Efficiency Formula:
Energy Cost Calculation
Application: Pumping water for irrigation
- Flow rate: 60 L/s (216 m³/h)
- TDH: 50 m
- Pump efficiency: 78%
- Motor efficiency: 94%
- Operating hours: 2,000 h/year
- Energy price: $0.25/kWh
Step 1 – Hydraulic Power (WHP):
Step 2 – Shaft Power (BHP):
Step 3 – Input Power (Motor):
Step 4 – Annual Energy Cost:
Impact of Efficiency on Costs
If the same duty were served by a less efficient pump (65% instead of 78%):
New input power
≈ 48 kW
Annual cost
≈ $24,000
The Cost of Inefficiency
That’s a difference of $4,000 every year. Over 10 years, the energy penalty exceeds $40,000 — far more than the initial purchase price of the pump.
Improving Pump Efficiency
Select Pumps Near BEP
Avoid oversizing or undersizing
Variable Speed Drives (VSDs)
Match pump speed to demand instead of throttling
Reduce Friction Losses
Use larger, smoother pipes and minimise fittings
Regular Maintenance
Impeller wear, misalignment, and clogged strainers reduce efficiency
System Redesign
Sometimes a two-pump arrangement (duty/assist) is more efficient than one oversized pump
Motor Selection
Use premium efficiency IE3/IE4 motors
Life Cycle Costing (LCC)
Typical cost breakdown over a pump’s lifetime shows why efficiency optimisation is crucial:
Purchase Price
5–10%
Maintenance
10–15%
Energy Consumption
75–85%
Why This Matters
Even a 2–3% efficiency improvement can save more money than the pump itself costs over its lifetime.
Common Mistakes
Focusing only on initial purchase price instead of life cycle cost
Oversizing pumps “just in case” → leads to throttling losses
Ignoring motor efficiency and assuming it is constant
Underestimating operating hours or energy tariff changes
Failing to recheck pump performance after wear or system modifications
Summary
Pump efficiency has a direct and lasting impact on operating costs. By selecting pumps to operate close to BEP, using efficient motors, and designing low-loss systems, significant savings can be achieved. Energy cost analysis should always be part of pump selection, ensuring not only reliable performance but also long-term economic operation.