Solar irrigation: sizing a pump for your farm

Solar pumping suits farming better than almost any other solar application, for one reason that is easy to miss: you can store water instead of electricity. A raised tank is a fraction of the cost of a battery bank of equivalent usefulness, and it does not degrade.

Get the sizing right and a solar pump runs for years with no fuel bill. Get it wrong and it disappoints from the first week. The difference comes down to three numbers.

The three numbers

1. Total head

Head is the vertical distance the pump must lift water, measured in metres, and it is the number most often got wrong.

Total head is not just the depth of your borehole. It is:

• Static head — the vertical distance from the water level (not the ground level) to your discharge point. If the water sits 40m down and your tank inlet is 8m above ground, that is 48m.
• Drawdown — when pumping starts, the water level in a borehole falls. Ask your driller for the drawdown figure; ignoring it is a common and expensive omission.
• Friction loss — resistance in the pipe. It rises with flow rate and pipe length, and falls sharply with pipe diameter. A long run of narrow pipe can add many metres of equivalent head.

Add all three. That total is what the pump must overcome.

2. Daily water requirement

How many cubic metres per day do you need? For irrigation this depends on crop, area, soil and season. Your agricultural extension officer or an irrigation designer can give you a figure per hectare for your crop.

Size on your peak season requirement, not the annual average. A pump that copes in April and fails in the dry season has not solved your problem.

3. Water source

Borehole needs a submersible pump, sized to fit the bore diameter — usually 4 inches — and rated for the head.

River, dam or shallow well can use a surface pump, which is cheaper and far easier to service because it is not down a hole. But surface pumps are limited to about 8 metres of suction lift by atmospheric pressure, regardless of what the motor can do. Below that, you need a submersible.

Flow and head trade against each other

A pump does not have a single output. It has a performance curve: as head increases, flow decreases.

This is why "a 5.5kW pump" tells you almost nothing. The same pump might deliver a large flow at 30m head and a trickle at 120m. What you need is the point on its curve where your head meets your required flow.

Any supplier specifying a solar pump should be reading a curve, not matching a kilowatt rating to a budget. If they cannot show you where your operating point sits on it, they are guessing.

Sizing the array

Solar pumps are usually specified with a recommended array size, and it is generally larger than the motor rating — often 1.3 to 1.5 times.

That is deliberate, not oversizing. Panels only reach rated output in full midday sun. A larger array means the pump starts earlier, stops later, and keeps running through passing cloud. Pumping hours matter more than peak flow, because you are filling a tank over a day.

Store water, not electricity

This is the design decision that makes solar irrigation economic.

Solar-direct pumping runs the pump only while the sun shines, filling an elevated tank. Water is then available under gravity at any hour — for irrigation, for livestock, for the house.

Battery-backed pumping lets you pump at night, but you are paying for batteries to move water you could have moved for free at noon and stored in a tank.

Tank storage is usually the right answer. Size it for two to three days of requirement, so a run of overcast weather does not stop irrigation. A tank costs less than the equivalent battery bank, lasts longer, needs no management system, and will not degrade with cycling.

Batteries earn their place only where night pumping is genuinely unavoidable — some livestock operations, or where a pressurised supply must be available around the clock.

Controllers, drives and protection

A solar pump needs electronics between the panels and the motor.

DC pumps use a matched controller from the same manufacturer. Simple and efficient at smaller scales.

AC pumps need a solar pump inverter — sometimes called a drive — sized to the motor. Many are hybrid, meaning they can draw from mains or a generator when solar is insufficient, which is useful during a critical irrigation window.

Whichever you use, dry-run protection is not optional on a borehole. A submersible pump relies on the water around it for cooling. Run it dry for a few minutes and you have destroyed it. Protection comes from a well probe sensor, a water-level switch, or a float switch in the source — an inexpensive component that prevents the most expensive failure in solar pumping.

Also fit surge protection. Boreholes are often in open ground with long cable runs, which is close to ideal conditions for lightning-induced surges.

The mistakes that cost the most

Measuring head from ground level, not water level. This under-specifies the pump, and it will never deliver the expected flow.

Ignoring drawdown. The static level is not the pumping level.

Pipe too narrow. Saving money on pipe diameter costs you head — permanently, every day the pump runs. Larger pipe over a long run is almost always the cheaper decision over the life of the system.

No dry-run protection. The single most common cause of a destroyed borehole pump.

Sizing on average rather than peak demand. The pump must cope in the dry season.

Combining pumping and household supply into one system. They want different designs — a pump wants a big array and no battery, a house wants a modest array and storage. Combining them usually compromises both. Two separate systems are frequently cheaper and always better.

What to send us for a quotation

To specify a pump properly, we need:

1. 1Water source — borehole (with depth, static water level, drawdown and casing diameter), river, dam or shallow well
2. 2Total head — or the measurements to calculate it, including the height of your tank inlet
3. 3Daily water requirement in cubic metres, at peak season
4. 4Pipe run — length and diameter
5. 5Whether mains or a generator is available at the site
6. 6What the water is for — irrigation, livestock, domestic, or a combination

With those, the shortlist is straightforward and honest. Without them, any figure you are quoted is a guess.

Our pump range covers submersible and surface pumps, controllers and drives, and the protection hardware that keeps them alive. Send the six numbers above through the quotation form and an engineer will match a pump to your actual duty point rather than to a budget.