On a farm, a water treatment works or a commercial property, pumping is frequently the single biggest line on the electricity account. It is also the load with the most waste built into it, because most pumps were selected with a safety margin on top of a safety margin and then throttled back to suit. Here is where the money actually goes and how to get it back.
Why pumps waste so much energy
Three compounding habits are responsible for most of it.
- Oversizing at selection. The designer adds margin, the contractor adds margin, the supplier rounds up to the next frame size. The result is a pump that never runs near its best efficiency point.
- Throttling to compensate. The usual fix for an oversized pump is a partly closed valve. That valve converts electricity into heat and noise. The pump still draws the power; you simply throw away part of the result.
- Undersized pipework. Friction loss rises with roughly the square of flow. Every metre of head lost to friction is a metre the pump pays for, every hour, for thirty years.
The measures, ranked by typical saving
| Measure | Typical saving | Payback |
|---|---|---|
| Fit a VSD on a varying-demand duty | 25 – 50 per cent | 1 – 3 years |
| Right-size or replace an oversized pump | 15 – 30 per cent | 2 – 4 years |
| Trim the impeller to the actual duty | 10 – 25 per cent | Under 1 year |
| Increase mainline pipe diameter | 5 – 20 per cent | 2 – 6 years |
| Restore worn wear rings and impellers | 5 – 15 per cent | Under 1 year |
| Remove throttling by resizing instead | 10 – 20 per cent | 1 – 2 years |
| Upgrade to an IE3 or IE4 motor | 2 – 5 per cent | 3 – 6 years |
| Shift pumping to off-peak tariff hours | 10 – 30 per cent of cost | Immediate |
| Fix leaks in the distribution system | Varies, often large | Immediate |
| Clean or replace fouled pipework | 3 – 10 per cent | Under 1 year |
| Add solar input to the drive | Up to 100 per cent of daytime energy | 3 – 5 years |
Variable speed drives: the affinity laws in practice
The reason a VSD saves so much is arithmetic rather than magic. For a centrifugal pump, flow varies with speed, head varies with the square of speed, and absorbed power varies with the cube of speed.
| Speed | Flow | Head | Power |
|---|---|---|---|
| 100 per cent | 100 per cent | 100 per cent | 100 per cent |
| 90 per cent | 90 per cent | 81 per cent | 73 per cent |
| 80 per cent | 80 per cent | 64 per cent | 51 per cent |
| 70 per cent | 70 per cent | 49 per cent | 34 per cent |
So a pump slowed to 80 per cent speed delivers 80 per cent of the flow for about half the power. Any duty where demand varies – irrigation blocks, building water pressure, process transfer, tank filling against a changing level – is a candidate. Duties that are genuinely constant, such as a borehole filling one tank at one flow, gain much less, and there a correctly sized fixed-speed pump is often the better buy. The mechanics are covered in how VSDs improve pump efficiency.
Impeller trimming: the cheapest win on the list
If a pump is permanently throttled to hold down pressure, it is producing head you are deliberately destroying. Machining the impeller down to the diameter that produces the head you actually want removes the waste permanently, costs a fraction of a new pump, and needs no control gear.
The limits: most impellers can be trimmed by 10 to 20 per cent of diameter before efficiency falls away, and it is a one-way change. Trimming suits a duty that is fixed and known; a VSD suits one that varies.
Fix the system, not just the pump
Pipe diameter
Friction loss scales steeply with velocity. Going from 50 mm to 63 mm pipe on a long mainline can halve friction head. On a pump running 3 000 hours a year, the pipe pays for itself and then keeps paying.
Fittings and valves
Every elbow, tee and partly open valve costs head. Long-radius bends instead of sharp elbows, full-bore valves instead of gate valves, and one fewer fitting wherever the layout allows all add up on a complex system.
Leaks
Water lost to leaks was pumped at full cost. On older reticulation systems, non-revenue water of 25 to 35 per cent is common, which means a quarter of the pumping bill is buying nothing. Leak detection is often the highest-return energy project on a municipal or estate system even though it has nothing to do with the pump.
Maintenance is an energy measure
Worn wear rings let water recirculate inside the pump. The motor still pays for that water to be accelerated; it simply comes straight back. A pump with 3 mm of ring clearance instead of 0.3 mm can lose 10 to 15 per cent of its efficiency while appearing to work perfectly well.
Similarly, scaled or biofouled pipework quietly raises system head year on year. Monthly current logging catches both trends early – see signs your water pump needs maintenance for the full checklist.
Use the tariff
Where a site is on a time-of-use tariff, moving pumping out of the morning and evening peaks is pure profit – the energy is the same, the cost is not. It usually requires nothing but additional storage and a timer or a level controller. A reservoir sized for a day of demand converts a peak-hour load into an off-peak one and gives you load-shedding resilience at the same time.
A worked saving
A commercial property in Midrand runs a 15 kW booster set for domestic water, throttled to hold 4.5 bar, 18 hours a day.
- Current consumption: 15 kW at roughly 88 per cent load, 18 hours, 365 days = about 86 000 kWh a year
- Fitting a VSD with pressure control brought average speed to 78 per cent: absorbed power fell to about 48 per cent
- New consumption: roughly 42 000 kWh a year
- Saving: 44 000 kWh, worth well over R 100 000 a year at current commercial tariffs
- Installed cost of the drive and sensor: recovered inside the first year
Where to start
- Meter the pump. You cannot manage what you have not measured – a clamp meter and a notebook is enough to begin.
- Find the duty point. Measure actual flow and pressure and compare it to the pump curve. If you are far left of the best efficiency point, the pump is oversized.
- Check for throttling. A valve that is never fully open is a bill you are paying by choice.
- Decide between trimming and a drive, based on whether the duty varies.
- Then look at the pipework, the leaks and the tariff.
If the pump is old as well as inefficient, run the numbers on replacement rather than retrofit – when to repair or replace a water pump covers that decision, and our VSD and control products cover the hardware.
Want to know what your pumping is really costing? We carry out pump efficiency audits – duty point measurement, energy modelling and a costed list of improvements. Contact us or call +27 (15) 516-4991.