A solar water pump for a farm is sized from three numbers: the litres you need each day, the total head the pump must lift against, and the sunshine your own site receives. Get those three right and the array size falls out of arithmetic. Solar pumping suits Ugandan farms because it turns a fuel bill into a one off purchase, but it delivers a daily total rather than flow on demand, so a tank sits in every working system.
The selection engineering behind flow, total head, friction losses and the suction lift limit belongs on choosing an irrigation pump, which also sets out honestly what solar gives up against an engine. This page assumes you have read that and have decided to price a solar system. What follows is the part that page does not carry: how an array gets sized against your head and flow, why the tank replaces batteries, what changes between a borehole and a river, how to keep the panels on your farm, and what actually breaks.
What Sizing a Solar Pump for Your Farm Starts From
Every honest solar quote is built backwards from water, not forwards from panels. The physics is fixed and you can check it yourself. Lifting one cubic metre of water through one metre takes about 9.81 kilojoules, which works out at roughly 0.0027 kilowatt hours. Multiply cubic metres by metres of total head, multiply by that figure, and you have the hydraulic energy your crop needs each day.
Twenty cubic metres a day against twenty metres of total head comes to about 1.09 kilowatt hours of hydraulic energy. That is the whole job, expressed as electricity, and it is a small number. What inflates it is the losses between the panel and the water.
Those losses have a name and a range. FAO's global review of solar irrigation records that the first generation of photovoltaic pumping systems, using centrifugal pumps driven by direct current or variable frequency motors, ran at hydraulic efficiencies of 25 to 35 per cent. Positive displacement, progressive cavity and diaphragm pumps came later for smaller volumes and do better. So a third of what the array produces reaching the water is a reasonable planning assumption for a centrifugal solar set, and a supplier quoting you an array without stating the efficiency they assumed has not sized your system. That single question separates a real calculation from a guess.
Sunshine on Ugandan Farmland Varies More Than Farmers Expect
The last step in that worksheet is where most solar sizing quietly goes wrong, because there is no single Ugandan sunshine figure to divide by. NASA's long term climatology, taken at five points across the country, puts all sky irradiance at the surface between 4.81 and 6.02 kilowatt hours per square metre per day as an annual mean. Since one sun is defined as a kilowatt per square metre, that figure is also the number of peak sun hours the site gets, which is exactly what array sizing needs.
| Point | Sun hours | Annual rain | Reading |
|---|---|---|---|
| Moroto | 6.02 | 723 mm | Most sun, least rain |
| Gulu | 5.87 | 1,084 mm | Strong sun, dry Jan |
| Kampala | 5.34 | 1,394 mm | Middling both |
| Mbarara | 5.04 | 1,091 mm | Flat year round |
| Kabale | 4.81 | 1,124 mm | Least sun, most cloud |
Read the two columns together and the pattern is useful. Karamoja has a quarter more sunshine than the southwest highlands and roughly half the rain, so it is the part of Uganda where a solar pump has both the strongest case and the least alternative. Kabale has the weakest sun in the country and the least need for irrigation. A quote built on a national average overstates the array Moroto needs and understates the array Kabale needs, by about a fifth either way.
Size the Array on Your Weakest Month, Not the Year
Annual means hide the number that decides whether your crop survives. At Kampala the monthly climatology runs from 5.87 kilowatt hours per square metre per day in February down to 4.85 in June and 4.87 in July. The pattern repeats at every point measured: sunshine peaks between January and March and bottoms out in June and July.
That produces a split that nobody selling solar tends to mention. Uganda's main dry selling window, December into February, lands on the sunniest weeks of the year, so a solar pump is at its strongest precisely when dry season vegetable prices are at their best. The second dry window, June into August, lands on the year's weakest sunshine. A system sized off February output and run in July will deliver roughly fifteen per cent less water on a clear day, and considerably less than that under the cloud that suppresses those months in the first place.
So the design rule is blunt. If you intend to irrigate in both dry windows, size on June, not on February, and accept that the array will be oversized for December. If you only intend to sell into the December window, you can size on the better months and save money, but write that limitation down so you do not plan a June crop you cannot water. The irrigation water requirement calculation is where the daily volume for either choice comes from.
Turning Head and Flow Into an Array Size for One Acre of Irrigation
Run the whole thing through for a real case. One acre of dry season vegetables in central Uganda, applying four millimetres a day, needs about 16,200 litres, or 16.2 cubic metres. Say the water sits twelve metres down, the tank stand is three metres high, and friction plus emitter pressure adds another five, giving twenty metres of total head.
Hydraulic energy comes to roughly 0.88 kilowatt hours a day. At a third wire to water, the array must produce about 2.9 kilowatt hours. Divided by Kampala's 5.34 sun hours, that is an array of a little over half a kilowatt, before the installer's derating. Divide by Kabale's 4.81 and the same duty needs about a tenth more panel.
Now check that against what small systems actually contain. FAO's review lists a small scale submersible reference system delivering up to 12,000 litres a day on 260 watts of panel, and a complete Kenyan package built around a submersible pump with 300 watts of panel on a secured three metre stand, a controller, filters and one acre of drip. Twelve thousand litres covers about three quarters of an acre at four millimetres a day. So the honest reading is that a few hundred watts of panel irrigates most of an acre of vegetables in good sun, and anyone quoting you multiple kilowatts for a single acre is either pumping from much deeper or selling you array you do not need. The full cost stack for the acre sits on irrigation system cost per acre.
The Tank Is the Battery on a Solar Farm Water System
Solar irrigation almost never uses batteries, and the reason is arithmetic rather than fashion. Water is already an energy store. Lift it into a tank during the day and you have banked the morning's sunshine in a form that costs nothing to keep and never needs replacing. Batteries add capital, wear out on a cycle count, and store the same energy less durably.
There is a second reason, and it is the one that actually forces the tank into the design. Because a solar pump's output pressure and flow both track the sunshine, feeding a drip system straight off the pump does not work properly. FAO states this plainly: direct connection of drip to a solar pump is problematic, because the pressure the emitters need is not steady. The tank breaks that link. It fills at whatever rate the sun allows and discharges at a constant head set by its height.
Be ready for the price of that. The same FAO review notes that the tank is an additional expense and is often more expensive than the pump itself. Budget it from the first quote, not as an afterthought, and read farm water storage tanks for sizing and foundations before you commit to a volume.
Borehole Against Surface Water for a Solar Pump on the Farm
The water source changes the pump, the risks and the failure list. A borehole takes a submersible unit hung down the hole on cable and rising main, which is the only option once the water sits below the suction limit of a surface pump. A river, a lake or a reservoir takes a surface set on the bank or a floating unit, which is cheaper to install, easier to service and easier to steal.
Solar gives boreholes one genuine advantage that engines do not. Because a solar pump delivers a lower flow spread over more hours, abstraction is gentler on the aquifer, and FAO records reduced risk of borehole collapse as a result. A diesel set hammering a marginal hole at full rate for three hours does more damage than a solar set drawing the same daily volume across eight. Permitting, drilling, test pumping and water quality all sit on boreholes for agricultural irrigation, and the permit trigger is a motorised pump, which a solar pump is.
Surface water brings its own quirk worth knowing. Panels mounted on floats over a reservoir run cooler and so produce slightly more, while shading the surface underneath cuts evaporation and slows algae. FAO lists all three effects. Whether the arrangement is available and repairable in your district is a separate question, but if you are already building the water body described on farm reservoir construction, it is worth asking about.
Mounting and Theft: Keeping Solar Panels on a Ugandan Farm
Panels are the most portable valuable object on most farms, and FAO's review names theft as a frequently reported problem, serious enough that solar irrigation systems are often not insurable, which in turn blocks the loan finance that would have paid for them. That is a real cost of ownership, not a rural anecdote, and it belongs in the plan rather than in the regrets.
One trend is running in your favour. As panels get larger and cheaper and array voltages rise, a stolen farm panel is less useful for a domestic lighting system, so the second hand market for them is thinner than it was.
What Fails on Solar Irrigation Systems
Solar pumping has few moving parts, which is true and is not the same as trouble free. The failure list from FAO's review of field problems is dull and electrical, and almost every item on it is preventable.
There is also a safety point that gets skipped. FAO's early project record names dangerous direct current voltages as a hazard. A solar array is live whenever the sun is on it and cannot be switched off at source, so wiring work on the array side is not a job to improvise.
The Free Fuel Problem on Solar Irrigation Farms
The strongest argument for solar pumping is also the source of its main risk, and FAO says so directly. Once the fuel is free, the marginal cost of running the pump for another hour is nothing, and that leads to wasteful water use, over abstraction of groundwater and low application efficiency in the field. Some farmers end up selling water to neighbours at a profit, which is a rational response to a pump that costs nothing to run and a terrible outcome for the aquifer.
On a diesel set, the fuel bill is the discipline. On solar you have to supply the discipline yourself. Two things do it. Meter the water, or at least fill a tank of known volume so you know what you applied rather than guessing from how long the pump ran. And pair the array with an application method that does not waste what it delivers, which is why FAO keeps pairing solar with low pressure drip rather than open furrows; drip irrigation explained covers why that combination holds up.
The regulatory side matters too, and it is not optional because the fuel is free. Taking groundwater with a motorised pump sits inside Uganda's abstraction permit system regardless of what drives the pump, and a solar array counts as motorised.
Where a Solar Pump Is the Wrong Choice on a Farm
Four cases, stated plainly, because a page that only argues for solar is not worth reading.
Rented land with a short lease. Solar is a capital asset bolted to a specific plot. If you may not be farming that ground in three seasons, an engine pump you can carry away is the better buy, and petrol against diesel water pumps is the comparison you want instead.
A duty that needs high flow for short bursts. Flood irrigating a block, or filling a large reservoir quickly, wants a big engine for a few hours rather than a large array working all day. Solar is a marathon runner.
Deep water plus large area. Head and volume multiply. Lifting a lot of water from a long way down pushes the array into a size where the capital stops making sense against a grid connection, if one is within reach.
A site you cannot secure. If the panels cannot be watched, fenced or carried in, and you cannot insure them, the honest answer is that solar on that plot is a loan against theft. Fix the security question before the technical one.
Solar Water Pumps for Farms: Questions Farmers Ask
What does a solar water pump cost in Uganda? Prices move with the exchange rate, the shipment and the district, so treat any single figure with suspicion and collect at least three quotes for the same specified duty. What is stable is the shape of the bill. FAO's global review prices panels per watt installed and lists the pump, the controller, the cable run, the installation and the drip package as separate lines, and it notes that the storage tank is frequently more expensive than the pump. Ask for the quote itemised that way, because a lump sum hides which line has been cut to win the job.
How long before a solar pump pays for itself? That depends on what it replaces and how many days a year it runs, and the published answers vary too much to average. FAO's review gives payback of two to two and a half years for solar pumps in the Dakar to Saint Louis belt in Senegal, on a hectare of horticulture growing three crops a year for city markets with interest on the equipment. The same review finds Indian systems not economically viable against subsidised grid electricity unless utilisation exceeds two hundred days a year, which is rarely the case. Those two results are not in conflict; they show that annual running days and the alternative you are replacing decide everything. Work yours out on your own crop calendar.
Can a solar pump run my drip system directly? Not reliably. Pressure and flow both follow the sun, so emitter discharge wanders through the day. Pump into a raised tank and let gravity feed the drip at a steady head. That is the arrangement FAO describes and it is why the tank is not optional.
Do I need batteries? For irrigation, no. Store the energy as water in a tank instead. Batteries appear when the same array is also asked to run lighting or cooling at night, and FAO notes that using batteries in those multi use systems involves compromises on efficiency.
How many panels for one acre? There is no per acre answer, because head matters as much as area. Work it the other way: litres a day, times metres of head, times 0.0027, divided by the wire to water efficiency, divided by your site's sun hours in its weakest month. For a shallow lift on one acre of vegetables, FAO's reference systems suggest a few hundred watts rather than kilowatts.
Will a solar pump work in the rainy season? Output drops under cloud, and June and July are the dullest months in every part of Uganda measured. Those are also months when you need less water. The mismatch that bites is a dry spell inside the rains, when demand rises while irradiance is still low, and the tank is what carries you through it.
Is the panel output on the label what I will get? No. Nameplate output is measured at test conditions that Ugandan midday heat does not match, and dust, shading, tilt and cable losses all take their share. Ask the installer what derating they applied and whether the array size they quoted is before or after it.
What maintenance does a solar irrigation system need? Wash dust off the panels, keep them unshaded as trees grow, check cable runs for rodent damage, keep the controller cool and dry, and service the pump on its own schedule. None of it is skilled work, and skipping the first two items is the commonest reason a system quietly delivers less every year.
Once you have your daily volume, your total head and your site's weakest month written down, you have a specification a supplier can price rather than a wish they can interpret. Take that sheet to several installers, ask each for the wire to water efficiency and the derating behind their array size, and compare like with like. Installers are listed under where to buy irrigation equipment. Everything the pump then has to feed, from filters to emitters, sits in the irrigation and greenhouses guide.