How to Size a Solar Pump System for a Borehole

VEICHI SI21 solar pump inverter

Sizing a solar borehole system is a matter of working backwards from the water you need. You start with flow and head, choose a pump and motor, then select an inverter and a PV array to match. This guide walks through the steps and the information needed for a quotation.

Step 1: Daily Water Requirement

Decide how much water must be delivered per day, for example for livestock, household use or irrigation. Because solar pumping only runs while the sun is shining, the daily volume is delivered over roughly the sunny hours, so the flow rate while pumping must be higher than the daily average.

Step 2: Flow Rate

Daily volume divided by an assumed pumping period gives a preliminary average flow, not a guaranteed solar duty point. Pump output changes through the day, and peak-sun hours are an energy measure rather than a fixed number of full-output pumping hours. Check expected daily delivery using local seasonal solar data and the pump/drive performance, especially in the month with the least favourable balance of demand and sunlight.

The operating flow must also respect the borehole's tested sustainable yield. Increasing pump or array size cannot make an inadequate water source deliver more water sustainably.

Step 3: Total Head

Total head is the vertical lift from the water level in the borehole to the discharge point, plus friction losses in the pipe and any pressure needed at the outlet. Dynamic water level, which is the level while pumping, should be used and not the static level. A drawdown test or the driller's report helps here.

Step 4: Hydraulic Power and Motor Size

For water, an estimate of electrical power at a selected duty point is:

Electrical power (kW) ≈ Flow (m³/h) × Total head (m) ÷ (367 × combined pump and motor efficiency)

At 10 m³/h and 60 m total head, with combined efficiency of 0.50, the estimate is 3.27 kW. This is an energy estimate, not enough information to select a 4 kW motor or a particular pump. Select from the manufacturer's pump curve, absorbed-power curve, motor ratings and permitted operating range. If calculating PV power, also account separately for drive, cable and array losses.

Step 5: Choose the Pump and Motor

Select a pump whose curve gives the required flow at the required head, near its best efficiency point. Submersible three-phase AC motors are common for boreholes. Check the motor voltage, full-load current and the borehole casing diameter.

Step 6: Choose the Inverter

VEICHI SI21 solar pump inverter
VEICHI SI21 product example. Motor current, PV limits and site conditions determine the required model. View product listing.

Our solar pump inverter collection provides a starting point for comparing listed models against the motor nameplate.

The solar pump inverter must match the motor. Check:

  • Motor power and rated current, against the inverter rating
  • Motor voltage, for example 380 V three-phase
  • The DC input range and recommended Voc range

Step 7: Design the PV Array

Check array operating voltage (Vmp), cold-corrected open-circuit voltage (Voc), current and power against the exact drive's limits. Hot panels can have an operating voltage too low for the required motor output even when their nameplate Voc appears suitable.

For illustration, 13 panels with a nameplate Voc of 49 V total 637 V at standard test conditions. This does not establish that the string is suitable: the designer must apply the panel's voltage temperature coefficient, minimum site temperature and tolerances, then check operating voltage and current. Parallel strings increase current rather than voltage.

Array power must be chosen for the required seasonal water delivery and the manufacturer's allowed PV input. Do not use a universal panel-to-motor sizing ratio. Include orientation, shading, heat, soiling, wiring losses and any permitted derating.

Step 8: Protection and Level Control

A borehole pump must never run dry. Typical protection and control includes:

  • Dry-run or low-level protection in the borehole
  • A float or level switch in the storage tank to stop the pump when the tank is full
  • DC isolators and overcurrent protection on the PV side
  • Surge protection and correct earthing
  • Suitable cable sizing for the DC and motor cables

Step 9: Enclosure and Site Conditions

The inverter and protection gear normally sit in an outdoor enclosure. Direct sun, heat, dust and condensation all matter. Shade the enclosure where possible, and size ventilation and protection to suit the site. See our article on industrial enclosures for more on outdoor cabinets.

Long Motor Cables and Submersible Motors

Browse solution packs for grouped equipment options. Review each pack's included components and exclusions against the project requirements.

Confirm motor-cable length, cable size, motor insulation suitability and the drive manufacturer's requirements for output reactors or dV/dt filters. Check the motor's minimum speed and cooling requirements. A larger drive does not remove these constraints.

A Practical Planning Checklist

  • Borehole depth, dynamic water level and yield
  • Required flow and daily volume
  • Total head to the discharge point
  • Existing pump and motor details, if any
  • Motor kW, voltage and rated current
  • Planned PV panel type and quantity, if known
  • Tank or dam arrangement and level control
  • Whether remote monitoring is wanted
  • Indoor or outdoor enclosure and site conditions

Technical Note

This article gives general guidance only. Pump selection, PV design, electrical protection, earthing and installation must be assessed for the specific site in accordance with manufacturer documentation and applicable standards, and carried out by competent persons.

Related catalogue examples: Solar Pump Inverters | Custom Control Panels. For an equipment enquiry, contact Softcore Group Industrial Solutions at info@softcoregroup.co.za.

Technical Resources

Use the documentation for the exact supplied model and revision. Family documents can cover ratings and options that do not apply to every unit.

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