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Is Your Irrigation Pump Wasting Power? How to Check Pumping Plant Efficiency

Weathered wooden pump house and well discharge valve pouring water into furrow-irrigated rows at golden hour

Two wells, same crop, same lift. One pumps an acre-foot for a few hundred kilowatt-hours. The other needs far more to move the same water, and nobody notices because the pump still starts every morning and the water still comes out. The difference shows up only on the power bill, slowly, one season at a time.

That difference has a name: overall pumping plant efficiency. It is the share of the electricity going into your motor that actually ends up lifting and pushing water. When it slips, every acre-foot costs more, even if your rate and your schedule never change.

Our post on ag electric rates and peak hours covered when to run a pump so each kilowatt-hour costs less. This one covers the other half of the bill: how many kilowatt-hours your pump burns for each acre-foot, how a pump test measures it, and the field signs that say it is time to check.

Quick Takeaways

  • Kilowatt-hours per acre-foot is the number to know. Extension specialists in California use a simple formula: kWh per acre-foot = 1.0241 x total dynamic head (feet) / overall pump efficiency.
  • A pump test measures four things: flow rate, pumping lift, discharge pressure, and energy going into the motor.
  • Know the grades. California’s Advanced Pumping Efficiency Program rates 60% and higher as excellent, 50% to 60% as good, and 49% or less as a pump that may need a retrofit. Submersibles usually run about 10 points lower.
  • The average pump has room to improve. Nebraska Extension found the average pumping plant in a statewide study used about 30% more energy than necessary.
  • Watch for drift between tests. Falling flow, falling pressure, a dropping water level, and longer run times to finish the same set are early signs.

What “pumping plant efficiency” actually means

A pumping plant is the whole package: motor, drive, and pump. Each piece loses some energy. A presentation from California State University, Fresno’s Center for Irrigation Technology, published by UC Agriculture and Natural Resources, breaks overall pump efficiency (OPE, sometimes called “wire to water” efficiency) into three parts multiplied together:

  • Motor efficiency: 88% to 96%
  • Transmission efficiency: 90% to 97%
  • Bowl (impeller) efficiency: the 60s to mid-80s

Their example: a good plant at 0.93 x 0.96 x 0.75 lands at about 67% OPE. Drop the bowl efficiency to 0.52 and the same motor and drive land at about 46%. The motor is rarely the problem. Nebraska Extension notes that about the only things that hurt electric motor efficiency are bad bearings or a motor far larger than the job needs. Most lost efficiency lives in the pump itself.

The useful work is called water horsepower: gallons per minute x total dynamic head in feet / 3,960. Total dynamic head (TDH) is the pumping water level plus the discharge pressure converted to feet. Every 1 psi equals 2.31 feet of head.

Energy cost per acre-foot: the math

The same Fresno State presentation gives the formula for electric pumps:

kWh per acre-foot = 1.0241 x TDH / OPE

Their worked example uses 400 feet of TDH and 60% OPE: 1.0241 x 400 / 0.60 = about 683 kWh per acre-foot. At an example price of $0.21 per kWh, that is about $143 to pump one acre-foot.

Now run the same formula for the same well with the pump at 45% OPE: 1.0241 x 400 / 0.45 = about 910 kWh per acre-foot. At the same example price, that is roughly $48 more for every acre-foot, before demand charges. Multiply by the acre-feet you pump in a season and the gap gets real.

Notice what is not in the formula: flow rate. The presentation points out that a 50 HP pump takes twice as long as a 100 HP pump to move an acre-foot if head and efficiency are the same. Energy per acre-foot depends on how high you lift the water and how well the plant converts power into lift.

What a pump test measures

A July 2014 technical advisory from California’s Advanced Pumping Efficiency Program (APEP) at Fresno State’s Center for Irrigation Technology explains that a tester measures at least four things:

  1. Water flow rate
  2. Pumping lift for a well, or inlet pressure for a booster pump
  3. Pump discharge pressure
  4. Energy input to the pumping plant

From those, the report calculates TDH, overall pumping efficiency, kilowatt-hours per acre-foot, cost per acre-foot, and percent of rated motor load. APEP notes that motor load outside 80% to 115% of nameplate suggests the motor is not matched to the pumping condition. On a well, the tester may run the pump 30 to 45 minutes to let the pumping level settle, then shut it off and read the recovered water level, generally after at least 15 minutes.

APEP’s sample report shows why the number matters. A well pump measured at 39% OPE used about 491 kWh per acre-foot in the cost analysis. The assumed retrofit at 65% dropped that to about 305 kWh per acre-foot, an estimated 37.9% cut in energy cost, and the same yearly water took about 750 pump hours instead of 1,000. APEP is clear that this is an estimate, and that the cost analysis may not include demand charges.

How to read your OPE

Overall pumping efficiency APEP rating Submersible equivalent
60% and higher Excellent About 50% and higher
50% to 60% Good About 10 points lower
49% or less May need a retrofit About 10 points lower

The 2014 advisory reported the average for all turbine pump tests in its database at about 53%. These are general guides, so talk with your pump service company before deciding on a repair.

In the Midwest you will see a different scale. The Nebraska Pumping Plant Performance Criteria say an electric pumping plant should deliver about 0.885 water horsepower-hours per kWh, based on an 88% efficient motor and a 75% efficient pump. A test reports your plant as a percentage of that standard. In Nebraska’s statewide study of 130 farmer-owned pumping plants, 15% beat the criteria, but the average plant ran at only 77%, meaning it used about 30% more energy than necessary. NC State Extension adds that minor repairs are usually justified when a system tests between 60% and 80% of the Nebraska standard.

Why pumps lose efficiency

APEP boils it down to two reasons: the pump is physically worn, or it is no longer matched to the job, for example when the water table has dropped and total lift is now above what the pump was designed for.

  • Sand wear. Nebraska Extension explains that sand wears impeller vanes and the seal area between impeller and bowl. Water slips back and gets pumped again, which adds work with no benefit.
  • Impellers out of adjustment. Worn impellers can sometimes be reset to restore the seal, but Nebraska warns this must be done by a qualified person who accounts for lineshaft stretch under load.
  • A changed system. Switching from gated pipe to a pivot, or to a lower-pressure package, without changing the pump can leave it running far from its best efficiency point.
  • Worn bearings, misalignment, and tired motors. The Fresno State presentation lists worn bearings, shaft and tubing out of alignment, and motors rewound multiple times. APEP notes a very old or repeatedly rewound motor can be a significant contributor to low efficiency.
  • A deeper pumping water level. In the presentation’s example, a 75 HP well pump went from a 190-foot to a 230-foot pumping level. Flow fell from 650 to 510 gpm, discharge pressure fell from 40 to 22.5 psi, and OPE fell from 62% to 48%.

That last case is why water level and pump performance belong together. Our guides on drawdown and pump level and well specific capacity help sort out whether the well or the pump is losing ground.

Signs your pump is wasting power

You do not need a test to notice a pump drifting. You need a baseline and a habit of looking. NC State Extension’s troubleshooting guide pairs flow and pressure like this:

  • Low pressure and low flow: points to excessive pump wear. Have the internal parts evaluated.
  • High pressure and low flow: the discharge may be partly blocked, or too few outlets are open.
  • Low pressure and high flow: too many sprinklers running at once, or worn nozzles.
  • Normal pressure and flow, but low efficiency: usually a drop in water level at the source or a blocked intake.

Add a few field signs to that list:

  • Sets take longer. If flow drops, the same acre-inches take more hours. APEP’s sample retrofit moved the same yearly water in 750 hours instead of 1,000.
  • The well’s water level keeps dropping. More lift means more kWh per acre-foot, straight from the formula.
  • The bill climbs but the water does not. Kilowatt-hours rising faster than acre-feet pumped is the clearest sign of all.

Nebraska Extension also notes that if you keep records of total energy use, water pumped from a meter reading, discharge pressure, and pumping water level, you can estimate your performance rating as a first screen before calling a technician.

When to test, and what to do with the results

  1. Test new pumps. APEP recommends a test on a new pump to set a baseline and confirm it runs as designed.
  2. Set a schedule by use. APEP suggests testing every one to three years depending on use and conditions: every year for a well pumping a lot of sand, every two to three years for a booster on clean water. Texas A&M AgriLife’s vegetable growers handbook suggests at least every five to eight years, so lightly used pumps can stretch the interval.
  3. Test in season. Results only hold for the flow and lift measured. APEP notes it is probably better to test an irrigation well during the growing season, not winter.
  4. Compare against the pump curve and past tests. Keep every report and your original pump curve together.
  5. Start with adjustments. In Nebraska’s tests, 58% of pumping plants could potentially benefit from adjustments, and field adjustments made with a wrench saved an average of 14% on energy costs.
  6. Ask about test programs. Texas A&M notes some electric utilities and groundwater conservation districts test pumps at no charge. Check with your utility, district, or pump dealer about what is offered in your area.

The catch: a test is a snapshot. The Fresno State presentation says it plainly. Flow from a well can change a lot through a season, and a test will not help if something starts to go wrong mid-season. That gap between tests is where monitoring earns its place.

Questions growers ask

How do I calculate kWh per acre-foot for my irrigation pump? For an electric pump, kWh per acre-foot = 1.0241 x total dynamic head in feet / overall pump efficiency. At 400 feet of head and 60% efficiency, that is about 683 kWh per acre-foot. Multiply by your cost per kWh for an energy cost per acre-foot, keeping in mind that demand charges are separate.

What is a good overall pumping plant efficiency? California’s APEP rates 60% and higher as excellent, 50% to 60% as good, and 49% or less as possibly needing a retrofit. Pumps with submersible motors usually run about 10 points lower in each category.

What does an irrigation pump test measure? Flow rate, pumping lift (or inlet pressure on a booster), discharge pressure, and energy input to the motor. From those the tester calculates total dynamic head, overall efficiency, kWh per acre-foot, and cost per acre-foot.

How often should I test my irrigation pump? APEP suggests every one to three years depending on use, and every year for wells pumping sand. Texas A&M suggests at least every five to eight years. Test new pumps for a baseline and test during the irrigation season.

Is pump efficiency the same thing as time-of-use savings? No. Time-of-use scheduling lowers the price of each kWh. Pump efficiency lowers how many kWh each acre-foot takes. See our post on ag electric rates for irrigation pumps for the scheduling side.

How AgriLynk Helps

AgriLynk does not replace a pump test or your pump company. It helps you notice drift between tests. Remote well monitoring brings well water level into one dashboard, and flow and pressure sensors can be added as expansion modules, so a dropping well water level or a falling flow shows up as a trend instead of a surprise. With pump and well control, configured alerts help your team react when pressure, flow, runtime, or other supported conditions fall outside expectations, and supported start, stop, and timed runs are set up after AgriLynk reviews your controller, panel, power, and safety requirements.

Field stations report every 8 minutes, and 12 months of dashboard history is included, so readings from the past 12 months are there when the tester shows up. A single remote well can start with a cellular field station where there is compatible cellular coverage. Several wells, pumps, or more than one farm are usually Enterprise / Farm Network setups. See pricing, tell us about your pump, or call (760) 723-1529.

Where supported, AgriLynk can also read your pump’s power and energy use from a compatible VFD or pump controller, so you can compare it with your test results over time. We confirm your specific equipment during quoting.

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