Sizing a Solar Borehole Pump: Panels, Depth and Daily Water Needs

Getting a solar borehole pump wrong is an expensive mistake. Undersize the panels and the pump starves for power on cloudy days; oversize the pump for the borehole’s actual yield and you pump it dry, wrecking the motor. This guide walks through the logic installers use so you can have an informed conversation — and spot weak proposals before you sign anything.

Why sizing is not a simple formula

You will find online calculators that spit out a panel count the moment you type in a depth. Treat those as a starting point only. Real sizing is iterative: depth affects required pressure, which determines motor wattage; motor wattage and daily run-hours determine panel capacity; and your household’s daily demand sets the run-hours target. Change any one input and the whole chain shifts.

For South African homeowners, a useful baseline is that the average person uses roughly 237 litres per day (IWA Water Policy figure), well above the world average. A family of four therefore needs around 950 litres daily from the borehole — before you account for irrigation, livestock or any other non-household use.

Step 1 — Know your borehole’s yield before anything else

Yield is the maximum sustainable flow rate your borehole can deliver without the water level dropping so far that the pump runs dry. It is measured in litres per hour or cubic metres per hour and is established by a proper borehole yield test. Without that number, every other calculation is guesswork.

A low-yield borehole (say, a few hundred litres per hour) may need a smaller, slower pump running longer rather than a powerful pump running briefly. Pushing a high-flow pump into a low-yield hole is one of the most common — and most costly — mistakes in DIY or underqualified installations.

Step 2 — Establish total dynamic head

Pumps are rated against total dynamic head (TDH), which is not simply the depth to the water surface. TDH adds:

• Static water level — the depth from surface to the resting water level.
• Drawdown — how far the level drops during pumping.
• Friction losses — resistance in the rising main and fittings.
• Delivery head — the height from ground level to wherever the storage tank sits.

A borehole with a static level of 30 m, 10 m of drawdown, 5 m of friction losses and a tank 5 m above ground gives a TDH of 50 m. That TDH, combined with the required flow rate, determines the pump’s hydraulic power requirement.

This is why the choice of pump model and the choice of pipe diameter happen at the same time — a wider rising main cuts friction losses and can allow a smaller, cheaper motor. Exploring those trade-offs is part of what you are paying an installer for. The borehole drilling overview explains what the broader installation process looks like from survey to commissioning.

Step 3 — Match motor size to your solar array

Once TDH and flow rate are fixed, the installer selects a pump with a motor rated in kilowatts (kW). That motor rating, plus an allowance for inverter or variable-speed-drive losses (typically in the range of 10–15 %, though confirm with your installer), gives the peak power draw.

Peak power draw divided by usable peak sun hours per day gives the minimum panel capacity in kilowatts-peak (kWp). South Africa’s solar resource is generally strong — Greater Johannesburg typically sees around 5–5.5 peak sun hours on a clear day, though this drops in winter and on overcast days. A system sized only for perfect conditions will underperform too often to be reliable.

Practical sizing therefore builds in a margin. As a rough guide, installers often add 20–30 % to the theoretical minimum panel capacity to cover cloud, panel degradation over time, and dust on the glass. Get that margin confirmed in writing in any proposal.

Because solar output varies through the day, most solar borehole systems include either a storage tank (the pump runs during daylight and water is stored for later) or a battery buffer — or both. Read the comparison of solar versus Eskom-powered pumps for a fuller look at when each approach makes financial sense.

If you want installers to run these calculations against your specific borehole and site, that is exactly what the quoting process is for. Compare 3 free quotes — vetted installers, no obligation.

Pump type matters as much as pump size

Submersible pumps are the norm for boreholes deeper than about 8–10 m. For solar applications, the two main motor configurations are:

Type	How it works	Best for
DC brushless motor	Runs directly from panel voltage via a controller	Simpler systems, shallower depths, smaller arrays
AC motor with solar inverter/VSD	Converts panel DC to AC; speed varies with available power	Deeper boreholes, higher flow rates, more flexibility

Variable-speed drives (VSDs) are increasingly popular because they allow the pump to slow down when sunlight is weaker rather than switching off entirely, which smooths daily output and reduces water-hammer stress on the rising main.

Choosing between these options — and choosing a motor size that will not burn out if the borehole drawdown is greater than expected — is covered in detail in the guide to choosing the right borehole pump size.

What to check in any installer’s proposal

A credible solar borehole proposal should show you, at minimum:

• The assumed static water level and estimated drawdown.
• The calculated TDH and the flow rate the system is designed to achieve.
• Peak sun hours used in the calculation and the margin applied.
• Panel wattage, quantity and total kWp.
• Motor rated power in kW and the brand/model of VSD or controller.
• Daily output estimate (litres per day) and the storage tank size.
• Water quality test recommendation — drinking-water installations should reference SANS 241 (2024 edition), the current South African standard.

If a proposal skips any of these, ask for them before signing. Vague proposals produce undersized or oversized systems, and the gap between the two is often R20,000 or more in either wasted hardware or future repair bills.

Proposals are easier to compare when you have more than one in front of you. Get up to three installer quotes — free, with no commitment required.

Costs: what the figures tell you

Complete borehole systems in Johannesburg typically run R40,000–R150,000, with most falling between R60,000 and R100,000. Borehole pumps alone typically cost R15,000–R35,000 installed. Solar adds panels, a VSD or inverter, cabling and usually a storage tank on top of those figures. If you are outside Johannesburg, treat those numbers as a reference point only — local drilling conditions, transport costs and installer density all shift the final price, so verified local quotes are essential.