A one-acre solar pump should be selected from a defined duty point: the flow required at the total dynamic head the system will actually impose. A maximum-flow number measured at low head cannot answer that question. Begin with water, not watts. Then check whether the source can supply that water, whether the pump can lift it during useful sunshine, and whether the irrigation system can distribute it evenly.
One acre is not one pumping job
A tomato field under drip, a maize plot under sprinklers and a nursery watered from a raised tank may occupy the same area but create different duties. Crop water changes with climate and growth stage. Gross water withdrawal changes again with effective rainfall and application efficiency. The lift changes with water level, tank height, pressure and pipe friction. Solar production changes through the day and season.
That is why a statement such as “one-acre pump” must be treated as a manufacturer’s intended-use boundary. Futurepump’s official FAQ, for example, describes its SE1 as suitable for up to one acre, while also stating a maximum flow of 1,600 litres per hour and total head of 15 metres. Those are separate boundaries, not a promise that 1,600 L/h is available at 15 m. A buyer still needs the curve showing flow at the measured head.
Step 1: turn crop demand into daily litres
The FAO method calculates crop evapotranspiration as reference evapotranspiration multiplied by a crop coefficient: ETc = ETo × Kc. Irrigation requirement then accounts for effective rainfall. For a quick dry-day screening calculation, use a locally justified millimetres-per-day figure, multiply by 4,047 litres per acre, and divide by the assumed irrigation efficiency.
| Planning depth | Net litres for one acre | Gross at 90% efficiency |
|---|---|---|
| 3 mm/day | 12,141 | 13,490 |
| 5 mm/day | 20,234 | 22,482 |
| 7 mm/day | 28,328 | 31,475 |
These are scenarios, not crop prescriptions. Rainfall may reduce the net requirement; hot, windy weather or a sensitive growth stage may increase it. An irrigation designer should use local weather data, the crop stage, effective rainfall, soil storage and an observed system efficiency. The tomato drip guide shows that process in detail.
Step 2: translate litres into flow at head
If the system must deliver 22,482 litres in six useful pumping hours, the average target is about 3,747 L/h. That already exceeds the headline maximum of the cited 1,600 L/h one-acre model. It does not prove solar is unsuitable. It means the chosen daily demand, schedule or product category does not fit that particular boundary. Options include a longer solar window supported by a correctly sized array, dividing irrigation across days only where agronomically acceptable, reducing losses, adding storage, or comparing a higher-capacity pump.
Now add head. Total dynamic head includes the vertical difference under operating conditions, delivery pressure and losses through pipe, fittings, filters and valves. For a borehole, use the pumping water level, not the drilled depth or static level. Read the TDH measurement guide before requesting quotes.
A supplier response should identify a specific flow at your TDH. “Maximum 3,600 L/h” without the head is incomplete. Likewise, panel wattage does not fix a hydraulic mismatch. More panel capacity may extend useful operating time or help under weaker irradiance, but it cannot change the mechanical and hydraulic limits of the pump.
Step 3: use water storage deliberately
Solar output and irrigation timing rarely match perfectly. A tank lets a pump work during sunshine while irrigation continues later. Futurepump’s FAQ explicitly says its pumps do not include batteries and suggests a tank for gravity pumping when there is no sun. That is manufacturer guidance, not a universal design: drip lines and sprinklers need enough pressure, and elevation provides only about 0.1 bar per metre of water height before losses.
Choose storage from the risk you need to cover. One full design day gives operational resilience but may be expensive. A smaller buffer can smooth passing cloud and zone changes. Include dead volume, overflow, tank-cleaning access and a float control that prevents waste. Never assume a tank’s nominal capacity is all usable at the required pressure.
A one-page brief for suppliers
- Crop, growth stage, planted area and irrigation method.
- Calculated net and gross litres per peak day, with the ETo/Kc source and efficiency assumption.
- Water source, tested sustainable yield and dry-season water level.
- Static lift, pumping lift, delivery height, pipe length/diameter, filter and pressure requirement.
- Required flow at TDH, intended pumping hours, tank volume and number of irrigation zones.
- Requested pump curve, panel configuration, controller protections, warranty process, spares and landed price.
Run the same inputs through the ShambaPump planning calculator. Its solar wattage is a rough screen only; the supplier must size the array to the actual pump/controller, local irradiance and seasonal duty.
Method, limitations and buying boundary
This guide uses area × water depth ÷ application efficiency for gross daily volume, then divides by useful pumping hours. It does not design emitters, friction loss, electrical protection, foundations or permits. It does not substitute for a pumping test or current local weather data.
Kenya’s Water Resources Authority states that water abstraction where works are involved is regulated through application, authorization to construct and a water permit. Requirements vary by category and source. Confirm the current process with WRA before constructing or abstracting; a product purchase does not grant a water right.
Frequently asked questions
What pump size is needed for one acre?
There is no safe acreage-only answer. Establish daily litres, pumping hours, TDH and source yield, and compare pump curves at that duty point. An acre with 3 mm/day demand is not the same job as an acre at 7 mm/day.
Can I irrigate directly from the pump?
Possibly, if flow and pressure match the irrigation zones throughout the solar day. A tank can decouple pumping from irrigation, but gravity height and pipe losses must still meet emitter or sprinkler pressure.