A pump efficiency test is the cheapest engineering study a water district ever buys, and the one most often filed unread. It measures how much of the electricity going into the motor comes out as water lifted, and it puts a dollar figure on the difference. This is the arithmetic, in the order the test report presents it.
Water horsepower
The useful work is flow times head: WHP = Q × TDH ÷ 3960, with Q in gallons per minute and total dynamic head (TDH) in feet. TDH is the pumping water level below the discharge gauge, plus the discharge pressure converted to feet (psi × 2.31), plus the friction loss in the column. A well pumping 900 gpm against 400 ft of TDH is doing 90.9 water horsepower of work.
Wire-to-water efficiency
Input power is measured at the meter or with a power analyzer on the motor leads, in kW. One horsepower is 0.746 kW, so overall plant efficiency (OPE) = WHP ÷ (kW × 1.341). On 95 kW input the well above runs at 71.4%. That single figure folds together the motor, the drive if there is one, the shaft and bearings, the bowl assembly and the column — which is why it is called wire-to-water.
| Wire-to-water | What it usually means |
|---|---|
| 70% and above | A well-matched, recently installed or recently rebuilt pump operating near its best efficiency point. |
| 60–70% | Normal service wear, or operation somewhat off the curve. Worth watching; rarely worth pulling the pump for. |
| 50–60% | Worn impellers or wear rings, a pump running well off its design point, a throttled discharge, or a lowered water level that pushed the pump up its curve. The energy penalty usually pays for a rebuild within a few years. |
| Below 50% | Something is wrong. Utility pump-test programs have historically flagged plants in this range as first priority. |
Energy intensity: kWh per acre-foot
Efficiency alone does not say what the water costs. The number that does is energy intensity: kWh/AF = kW ÷ (gpm × 60 ÷ 325,851), because 325,851 gallons make an acre-foot. The example well needs 573 kWh for every acre-foot. At 1,200 AF a year and $0.18 per kWh that is about $123,800 a year in energy. Note that a deeper pumping level raises kWh/AF even at constant efficiency — lift is work — which is why a plugging well costs money before anyone notices the flow dropping.
Recoverable cost
The recoverable figure on a test report compares the plant’s actual kWh/AF with what the same water would cost at a target efficiency. Against a 72% target the example well recovers only about $1,100 a year — it is already close. Against the same numbers at 55% efficiency the recoverable figure would be roughly $38,000 a year, and a bowl assembly rebuild pays for itself in well under two years. That comparison, not the efficiency percentage on its own, is what decides the work order.
Reading the report critically
- Was the flow measured (meter or pitot) or assumed? An assumed design flow makes every other number fiction.
- Was the pumping level read after the well stabilized, or a minute after start-up?
- Was input power measured, or calculated from nameplate amps and an assumed power factor?
- Is the pump on a VFD? Efficiency at reduced speed is legitimate, but compare it with the curve at that speed, not the 60 Hz curve.
- How does this test compare with the last one on the same well? A single test is a snapshot; the trend is the diagnosis.
Waterwell IQ reads the test report, extracts these values, checks the arithmetic against the engine, and puts the result on the well’s timeline next to every earlier test, the specific capacity trend and the pump curve. Try the numbers yourself in the calculator on the home page.
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Sources
- University of California Agriculture and Natural Resources. Energy and Cost Required to Lift or Pressurize Water, Pub. IG6-96. n.d. States 1.02 kWh to lift one acre-foot one foot at 100% efficiency. The 1.023 used on this site is the same constant carried to one more place through the conventional 3,960 water-horsepower divisor.
- NC State Extension. Pumping Plant Performance. rev. 2024. Source for water horsepower = gpm x TDH / 3,960.
- Center for Irrigation Technology, California State University, Fresno. Advanced Pumping Efficiency Program Policies and Procedures Manual. 2015. Wire-to-water testing program. Its thresholds are program eligibility rules, not universal grades: a pump that tested at or below 30% overall efficiency (20% submersible) gets no subsidized retest until it is retrofitted; at or below 50% (40% submersible) it qualifies for a retrofit calculation.
- University of Nebraska-Lincoln Extension. Is Your Pumping Plant Doing All it Can?, Tom Dorn. 2009. Nebraska Pumping Plant Performance Criteria. In a 130-plant study the average plant measured 77% of the criteria, about 30% excess energy.
- USDA Natural Resources Conservation Service. Complete Pumping Plant Evaluation for Water Savings, E449A. 2019.
Every link above was retrieved and read on 2026-09-09; the Advanced Pumping Efficiency Program link was replaced and read on 2026-09-30, after the program moved its pages to pumpefficiency.org. Paid standards are named by designation so you can check them against your own copy.