Guides · Off-grid and irrigation
Specifying a solar water pump, from the water you need
A solar pump is not bought by wattage. It is bought by how many cubic metres a day you need, how high the water has to be lifted, and how many hours of real sun you get in your worst month. Get those three numbers right and the array size follows. Get them wrong and the pump runs dry by eleven in the morning.
The three numbers that decide everything
| Number | What it means | Where to get it |
|---|---|---|
| Daily volume | Cubic metres per day, in the driest month, not the annual average | Your own stock or crop figures |
| Total head | Vertical lift plus pipe friction plus the height of the tank inlet | Measured on site, not estimated |
| Peak sun hours | The hours of usable irradiance in your worst month | A local irradiance table, not a summer average |
Step 1: Daily volume, in the worst month
Start from what the water is for, and use your own per-head or per-hectare figure rather than a generic one — the honest answer is that it varies too much by breed, crop and climate for a supplier to hand you a number.
The point that suppliers get wrong is the month. A system sized on an annual average is a system that fails in the dry season, which is exactly when it matters. Size on the month when your demand is highest and your sun is lowest, and accept that it is oversized for the rest of the year. The alternative is a system that is adequate for eight months.
Step 2: Total head, measured rather than guessed
Total head is not the depth of the borehole. It is:
- Static water level — where the water sits when the pump is off
- Drawdown — how far the level drops once you start pumping. On a weak borehole this can be many metres, and it is the figure most often ignored
- Vertical rise to the tank inlet — not to the tank base
- Pipe friction — a function of pipe diameter, length and flow rate. Add 10–15 per cent as a working allowance for a reasonably sized line; a long or undersized line costs much more
Drawdown is where dry-run failures come from. A pump that is specified on static water level alone will be running above the water surface within an hour of start-up. That is why a level probe and dry-run protection are not optional extras on a borehole installation — they are what stops the first hot afternoon from destroying the pump.
Step 3: Size the array from the energy, not from the motor label
Lifting water is physics, and the physics does not care what brand the controller is. One cubic metre lifted one metre takes about 2.725 watt-hours of hydraulic energy:
hydraulic energy (Wh) = volume (m³/day) × total head (m) × 2.725
Then divide by the efficiency of the pump, motor and controller together — a realistic figure for a solar submersible set is around 0.4 to 0.5 — and divide by the peak sun hours to get the array in watts-peak.
A worked example
| Daily volume | 30 m³ |
|---|---|
| Total head | 40 m |
| Hydraulic energy | 30 × 40 × 2.725 = 3,270 Wh |
| System efficiency | 0.45 |
| Electrical energy needed | 3,270 ÷ 0.45 ≈ 7,270 Wh |
| Peak sun hours (worst month) | 5 |
| Array required | 7,270 ÷ 5 ≈ 1,450 Wp |
Two things follow immediately. First, the array is usually larger than buyers expect, and it is larger than the number on the pump. Second, a 750 W pump and a 1,500 W array is not a mismatch — the pump is the load and the array is the supply, and it is the supply that determines whether the pump runs all day or only in the morning.
Use an MPPT controller rather than a simple direct connection. It tracks the array's maximum power point as irradiance and temperature move, which on a hot afternoon is the difference between a pump that keeps working and one that slows to a trickle.
Submersible or surface?
Submersible, in the borehole or well
For anything with meaningful head, or where the water surface is more than a few metres down. Quiet, protected from weather and theft, and the right choice for boreholes, deep wells and most livestock supply. Needs the borehole diameter, the water level and the drawdown to be known before it is ordered.
Surface, self-priming, at the water's edge
For rivers, canals, shallow wells and tanks, where suction lift is within the pump's limit — in practice about 7 m, and less in hot conditions at altitude. Cheaper and easier to service, but it needs a dry, shaded, secure place to stand, and a suction line that does not leak air.
The four mistakes that stop a solar pump working
- Sizing on the annual average instead of the worst month. The system works beautifully until the season when you need it.
- Ignoring drawdown. The pump is correctly sized and still runs dry, because the water level it was specified against is not the level it pumps at.
- Undersizing the array relative to the pump. A larger motor on a small array does not move more water; it just runs below its rated point and stops earlier in the day.
- No dry-run protection or level probe. One afternoon without water is enough to destroy a submersible pump that would otherwise have run for years.
What to send us for a quotation
- Daily volume required, and in which month
- Static water level, drawdown if known, and the height of the tank inlet
- Borehole or well diameter, if submersible
- Pipe run length and diameter, if you already have the line in place
- Whether the site is solar only or has mains backup
- Destination country and port
With those six answers we can quote a pump, controller and array as a matched set rather than as three components that have to be reconciled later — and tell you the honest answer when a smaller, cheaper kit would do the same job. See the pumps we supply.
Send the volume, the head and the port
Those three numbers are enough to start. You get a written quotation with the pump, controller and array matched to your site, plus MOQ, packing and lead time.