How this is calculated
Lifting water is one of the few household jobs where the energy is pure physics: volume times height times the weight of water times gravity. Every watt of panel above that pays for losses.
People, animals and irrigation add into one daily volume. Total dynamic head is the standing depth to water, plus drawdown while pumping, plus the rise to the tank inlet, plus pipe friction — here Hazen-Williams at C = 150 against the inside diameter you picked.
Hydraulic energy is cubic meters a day, times head in meters, times 1,000 kilograms per cubic meter, times 9.81 meters per second squared, divided by 3,600 for watt-hours. That is the floor no equipment can get under.
Two divisions follow. Wire-to-water efficiency gives the electricity the pump must be handed — the World Bank’s 2018 primer puts a well-matched pair at about 60 percent at its best operating point and lower everywhere else on the curve. Peak sun hours and the same primer’s 0.774 derating chain for dirt, heat, cabling and the power conditioner give the panel watts.
Worked example
Three people at 82 gallons plus ten head at 12 gallons is 366 gallons a day, from 120 feet of standing water, 15 feet of drawdown and 25 feet up to the tank. Two hundred feet of 1-inch pipe loses 0.31 feet, so head is 160 feet.
That is 1.385 cubic meters lifted 48.86 meters: 1.385 × 48.86 × 1000 × 9.81 ÷ 3600 = 184 Wh. At 60 percent the pump wants 307 Wh, or 73.2 watts across a 4.2-hour window; divide by 4.2 and 0.774 and the array is 94.6 watts.
Three people at 124 liters plus ten head at 45 liters is 822 liters a day, from 36 meters of standing water, 5 meters of drawdown and 8 meters up to the tank. Sixty meters of 32 mm pipe loses 0.07 meters, so head is 49.1 meters.
That is 0.822 cubic meters lifted 49.07 meters: 0.822 × 49.07 × 1000 × 9.81 ÷ 3600 = 110 Wh. At 60 percent the pump wants 183 Wh, or 61.1 watts across a 3.0-hour winter window; divide by 3.0 and 0.774 and the array is 78.9 watts.
Quick answers
How many solar panels does a water pump need?
Fewer than almost anyone guesses. The example above, a household and ten cattle from 160 feet of head, lands just under 95 watts.
Does a solar water pump need a battery?
No. The tank is the battery: water lifted during the day is energy already spent and kept, and it never loses capacity with cycling.
What is total dynamic head?
Standing depth, drawdown, the rise to the tank inlet and pipe friction. The World Bank treats friction as roughly 10 percent of the dynamic water level plus the discharge head on a first pass.
How big should the storage tank be?
Two to three days of demand, the World Bank’s 2018 guidance; its field survey found real tanks more often undersized than oversized.
Is a bigger pump or a bigger array the better fix?
Usually the array. More current pushes a helical pump into flow earlier and later, while a bigger pump raises the threshold before any water moves.
Why a tank beats a battery for water
A battery stores electricity and converts it back into pumping on demand. Every conversion costs something, and the battery ages whether you use it.
A tank stores the finished work. Water already sitting above the wellhead has no round-trip loss, no state of charge and no replacement clock.
It fails gracefully too: a dead battery bank stops the water tonight, while a full tank on a dead pump gives you days. The well pump off-grid calculator lands in the same place from the other direction.
Panel watts by daily volume and head
The same arithmetic at 60 percent wire-to-water and the World Bank’s 0.774 derate, with pipe friction left out. Read your row and column, then scale it.
| Water per day | 50 ft head | 100 ft | 150 ft | 200 ft | 300 ft |
|---|---|---|---|---|---|
| 100 gal | 8 W | 16 W | 24 W | 32 W | 48 W |
| 250 gal | 20 W | 40 W | 60 W | 81 W | 121 W |
| 500 gal | 40 W | 81 W | 121 W | 161 W | 242 W |
| 1,000 gal | 81 W | 161 W | 242 W | 322 W | 484 W |
| 2,000 gal | 161 W | 322 W | 484 W | 645 W | 967 W |
| 5,000 gal | 403 W | 806 W | 1,209 W | 1,612 W | 2,418 W |
| Water per day | 15 m head | 30 m | 45 m | 60 m | 90 m |
|---|---|---|---|---|---|
| 400 L | 12 W | 23 W | 35 W | 47 W | 70 W |
| 1,000 L | 29 W | 59 W | 88 W | 117 W | 176 W |
| 2,000 L | 59 W | 117 W | 176 W | 235 W | 352 W |
| 4,000 L | 117 W | 235 W | 352 W | 469 W | 704 W |
| 8,000 L | 235 W | 469 W | 704 W | 939 W | 1,408 W |
| 20,000 L | 587 W | 1,174 W | 1,760 W | 2,347 W | 3,521 W |
Peak sun hours vary by month and latitude far more than these columns do — take your worst-month figure from the peak sun hours atlas.
Helical and diaphragm pumps against an AC submersible on a controller
Positive-displacement pumps move a fixed slug of water per turn, so they keep lifting at low speed. The World Bank puts them at 30 to 250 meters of head and low flow — the shape of most well and livestock jobs.
The helical rotor suits weak light: it pumps slowly without losing efficiency, so a thin morning gives a thin trickle rather than nothing. Diaphragm pumps behave similarly at modest heads and cost less, but the diaphragm and valves are wear parts.
Centrifugal pumps, including AC submersibles on a solar controller, suit high flow and lower head, roughly 10 to 120 meters. They produce nothing below a threshold speed, so flickering cloud hurts them far more.
Why oversizing the array beats a bigger pump
A panel-direct pump never runs at one duty point. The array current at nine in the morning decides whether water moves at all.
Panel watts widen that window at both ends: the pump starts earlier, holds flow through thin cloud and finishes later, so daily volume rises without touching well or pipe.
A larger pump does the opposite. It raises the power the array must reach before the first drop arrives, and pushes the pair away from the point where that 60 percent lives.
Dry running, float switches and freeze protection
A dry run is the classic way to kill a solar pump. A helical rotor relies on water for lubrication and cooling, and a slow-recovering well hands it air. A well probe that cuts the controller, with a timed restart, is not optional.
A float switch stops the pump when the tank is full. Without one you spill the well over the ground all afternoon, which on a slow well guarantees that dry run.
Freezing gets the riser. A standing column above the frost line bursts the pipe, so panel-direct installations drill a small drain-back hole below frost depth, and the run empties whenever the pump stops.
Size the demand with the water numbers chart, then read water pressure without the grid for what happens between tank and tap.
What a linear current booster wants from the array
Wire a panel straight to a motor and the motor drags the array off its maximum power point exactly when light is scarce. A linear current booster, or the tracking inside a modern controller, trades surplus voltage for the current the motor needs to turn.
So array voltage is a design choice. The controller wants input voltage above what the motor asks for at the module temperature of a hot afternoon rather than the label figure, and open-circuit voltage on the coldest morning must stay under its ceiling.
Build a string that lives inside that window in both directions. Long runs argue for higher voltage and thinner copper, which the DC wire size calculator settles.
Sources
Sources: World Bank, Solar Pumping: The Basics, 2018 — head components, the hydraulic energy equation, 60 percent wire-to-water at the optimal operating point, the 77.4 percent derating chain, friction at 10 percent of the dynamic water level plus discharge head, the two-to-three-day tank rule, cattle at 40–50 liters a head a day, and head ranges of 30–250 m for positive displacement against 10–120 m for centrifugal.
NDSU Extension AS1763, Livestock Water Requirements, revised February 2026, for 11.4 gallons a lactating beef cow at 40°F rising to 16.9 at 70°F. EPA WaterSense, citing the US Geological Survey’s Estimated Use of Water in the United States in 2015, for 82 gallons a person a day; EurEau, 2022, for 124 liters. Friction is Hazen-Williams at C = 150 against schedule 40 inside diameters. Checked September 2026.
Go deeper
If the well already holds an AC submersible, panel-direct is a second pump rather than a conversion: compare both in the well pump off-grid calculator, check the yield with the well yield and storage calculator, then settle storage with off-grid water storage sizing.