How to Choose a Solar Water Pump for Irrigation

Start with daily water demand

Pump selection starts with the water the crop needs on the design day, not with motor wattage. Estimate crop demand from irrigated area, net irrigation depth and application efficiency:

daily water (m3/day) = area (m2) x net irrigation depth (mm/day) / 1,000 / application efficiency

Use local crop, soil and climate data wherever possible. The FAO crop evapotranspiration guidance is a useful basis when a farm does not already have an irrigation schedule. Allow for effective rainfall rather than treating every millimetre of crop demand as pumped water.

For an existing drip or sprinkler layout, also total the flow of the emitters that operate at the same time. That number is the flow required by one irrigation zone. Do not add emitters from zones that never run together.

Convert daily volume into pump flow

A solar pump normally has fewer productive pumping hours than daylight hours. Early morning and late afternoon irradiance may be enough to start the controller but not enough to deliver the design flow. Use a realistic operating window for the location and season:

required average flow = daily water volume / productive pumping hours

For example, 12 m3/day delivered over five productive hours requires an average of 2.4 m3/h. If the irrigation system must run directly from the pump, the active zone flow may be higher than this average and should govern the initial selection.

Add one stated allowance for seasonal variation and normal operating losses. Avoid adding a separate large margin to demand, flow, head and array power; stacked allowances can turn a sensible design into an oversized system.

Calculate total dynamic head

Total dynamic head (TDH) is the head the pump must overcome at the required flow. For a typical irrigation project:

TDH = pumping lift + elevation rise + required pressure head + pipe, fitting and filter losses

Head component	What to measure	Common mistake
Pumping lift	Vertical distance from the pumping water level to the discharge reference point	Measuring from the top of the well instead of the pumping water level
Required pressure	Pressure needed by drip lines, sprinklers or filters	Ignoring the minimum pressure at the field inlet
Pipe friction	Loss through pipe, fittings, valves and filters	Selecting pipe only by the pump outlet size

Use the pumping water level

For a borehole, use the expected water level while the pump is running. The standing level may be several metres higher. Designing from the standing level makes the calculated duty look easier than the duty the pump will actually see.

For a surface pump, suction conditions need a separate check. Suction lift, pipe losses, water temperature and the pump’s available net positive suction head affect whether the pump can operate without cavitation. Maximum head alone does not answer that question.

Include pressure as head

Irrigation equipment needs pressure at the point of use. One bar is approximately 10.2 metres of water head, so a field inlet requiring 1.5 bar adds about 15.3 metres to TDH before pipe and filter losses are included. The USDA NRCS Irrigation Guide provides a fuller treatment of pressure, elevation and friction in irrigation design.

Read the pump curve at the operating point

Once flow and TDH are known, mark that duty point on the model-specific performance curve. The curve, not the model name or motor power, shows whether the pump can supply the required flow at that head.

Do not compare pumps only by the maximum values printed in a catalogue. Maximum flow is usually measured near zero head, while maximum head occurs near zero flow. The useful operating range lies between those limits.

Check the full operating range

Water level, filter condition and irrigation zones may change during the season. Check at least three points: the normal duty, the highest expected head and the lowest expected head. Confirm that the controller and irrigation network can tolerate the resulting flow range.

Worked screening example

Consider a borehole project with the following measured or specified inputs:

Input	Design value
Daily water requirement	12 m3/day
Productive pumping window	5 hours/day
Required average flow	2.4 m3/h
Pumping water level below ground	10 m
Field inlet above the wellhead	6 m
Required inlet pressure	1.0 bar, approximately 10.2 m head
Estimated pipe, fitting and filter loss at 2.4 m3/h	4 m

The preliminary duty point is therefore 2.4 m3/h at about 30.2 m TDH. The friction allowance must be recalculated from the actual pipe material, internal diameter, route and fittings before ordering.

The published maximum values in the current Rakour solar water pump range can be used to eliminate models that clearly cannot reach the duty. They cannot confirm the final model by themselves.

Published model	Catalogue limits	Screening result
2DPC1.5-35-24-200	1.5 m3/h maximum flow; 35 m maximum head	Eliminate: maximum flow is below 2.4 m3/h.
3DPC3.5-35-24-300	3.5 m3/h maximum flow; 35 m maximum head	Candidate only: check whether its verified curve reaches 2.4 m3/h at 30.2 m.
3DSS1.7-100-48-500	1.7 m3/h maximum flow; 100 m maximum head	Eliminate: high head capability does not compensate for insufficient flow.
4DSC3.5-50-48-400	3.5 m3/h maximum flow; 50 m maximum head	Candidate only: verify the curve, electrical limits and required array.
DCPM15-14-48-550	15 m3/h maximum flow; 14 m maximum head	Eliminate: maximum head is below the calculated TDH.

This example intentionally stops at a shortlist. A catalogue maximum is a boundary, not a promise that maximum flow and maximum head occur together. If neither remaining curve crosses the duty point with a practical margin, the correct response is to change the system design or select another pump, not to force a catalogue model into the project.

Match the solar array and controller

After the hydraulic shortlist is established, match each candidate to its controller and solar array. Panel wattage alone is not enough. Check array operating voltage, open-circuit voltage (Voc), current, controller input limits and cable voltage drop.

Calculate the array’s cold-condition Voc from the module data sheet and the lowest expected cell temperature. Module Voc changes with temperature; the Sandia PV Array Performance Model describes the role of the open-circuit-voltage temperature coefficient. The complete series string must stay below the controller’s absolute Voc limit.

The array normally needs more rated power than the pump motor rating. The exact ratio depends on the controller, climate, installation angle and required operating window. Use the pump manufacturer’s array recommendation as the starting point.

Size pipework for the required flow

Long or undersized pipes increase friction loss and move the real duty away from the original calculation. A larger pipe can reduce head loss and the energy needed to move each cubic metre of water, but it should be chosen by a hydraulic calculation rather than by a simple rule of thumb.

Check the complete route, including rising main, horizontal delivery pipe, elbows, non-return valves, filters and manifolds. Pipe diameter should be chosen from required flow, route length and acceptable velocity, not only from the pump connection size.

Decide whether to store water

Water storage often makes a solar irrigation system easier to operate. The pump can fill a tank during daylight, while irrigation runs later at a controlled rate. This separates variable solar production from the field schedule and often avoids using batteries for water delivery.

A direct-pumping system can work when the active irrigation zone accepts the pump’s changing flow and the available solar window matches the watering schedule. The agricultural irrigation planning page covers the wider choice between source, storage, field pressure and delivery method.

Information to prepare before requesting a quotation

Record the assumptions, units and source of each input. A useful request for quotation includes:

1. Water source and expected pumping water level.
2. Vertical elevation from water level to the final discharge point.
3. Required daily water volume and target hourly flow.
4. Required pressure at the irrigation inlet.
5. Pipe length, internal diameter and important fittings.
6. Location, seasonal solar conditions and available panel area.
7. Water quality, including sand, sediment and expected temperature where relevant.
8. Proposed solar-module data sheet and the lowest expected site temperature.

Final selection checklist

Before accepting a quotation, confirm all of the following:

• The model-specific curve crosses the required flow and TDH.
• Seasonal water levels have been checked, not only the best-case level.
• Pipe and filter losses were calculated at the selected flow.
• Array operating voltage and cold-condition Voc remain inside the controller limits.
• Cable length and conductor size are included in the electrical review.
• The quotation states the assumed duty point and the documents used for selection.

If any of those items is missing, the selection is not finished. Send the measured site data through the project inquiry form so that each candidate can be reviewed against the same duty point.

Technical basis and scope

This guide was prepared by the Rakour Solar Pump Applications Team and technically reviewed on 20 September 2026. It combines the published Rakour product data linked above with established irrigation and photovoltaic design references.

It is a pre-selection method, not a substitute for the current model-specific performance curve, controller manual, site survey, local electrical rules or review by a qualified irrigation and electrical professional. No pump should be ordered from maximum-flow and maximum-head values alone.

References

1. Food and Agriculture Organization of the United Nations, Crop Evapotranspiration: Guidelines for Computing Crop Water Requirements, Irrigation and Drainage Paper 56.
2. USDA Natural Resources Conservation Service, National Engineering Handbook, Part 652: Irrigation Guide, particularly the irrigation system design guidance on pressure, elevation and friction losses.
3. Sandia National Laboratories PV Performance Modeling Collaborative, Sandia PV Array Performance Model, for photovoltaic voltage and temperature variables.
4. Rakour, Solar Water Pumps, for the product limits used in the screening example. Final selection requires the current curve and installation documentation for the exact model.