Deep-Well Solar Pump Selection: A Borehole Buyer's Guide

Selecting a deep-well (borehole) solar pump is a process of elimination against five hard constraints, in this order: the borehole's inner diameter, which fixes the maximum pump body size; the setting depth, which must respect both the pump's maximum immersion and the well's dynamic water level; the sand and water chemistry, which decides pump type and wetted materials; the duty point of flow at total head; and the cable cross-section for the drop length. A pump that fails any one of the first three will not survive the installation, no matter how well its hydraulics match.

This guide is written for buyers and installers specifying submersible pumps for wells rather than for surface water. Each constraint is explained with the specification you should demand from any supplier, and the concrete figures shown are taken from the published data for the ten Aquasolar deep-well series.

Constraint 1 — Borehole diameter comes first

A submersible pump has to physically pass down the casing, and there is no workaround. Two numbers matter, and they are not the same thing:

• Nominal size (2″, 3″, 4″, 6″) — a size class, not a measurement.
• Actual outer diameter (OD) in millimetres — the real dimension that must clear the casing's inner diameter, with room for the rising main coupling, cable, safety rope and any casing deformation or scale.

Always ask for the OD in millimetres and the stated minimum borehole size. Here is the published data across the ten Aquasolar deep-well series:

Two practical points follow from that table. First, the margin between pump OD and minimum borehole is small — 2 mm at 2″–4″, 6 mm at 6″ — so a nominally adequate borehole that is out of plumb, partially collapsed or scaled may still be too tight. Where the well log is old or unavailable, a dummy run with a test slug of the same diameter is cheap insurance.

Second, a narrow borehole restricts your choice of pump type, not just its size. In a 3″ casing you are choosing between a screw pump and a slim impeller pump; the high-flow 6″ hydraulics are simply unavailable. If the water requirement is large and the borehole is narrow, the honest answer is usually a second borehole rather than an optimistic pump selection.

The 2″ special case

A 50 mm pump in a 52 mm borehole is the tightest configuration in the range and delivers modest flow — the 2″ screw series is rated at 1.2–1.7 m³/h — but it reaches up to 140 m of head on 180–750 W. For hand-drilled or very narrow wells serving a household or a livestock trough, it is often the only option that fits, and its flow is adequate for that duty.

Constraint 2 — Setting depth, immersion and submergence

Three different depths get confused in tender documents, and mixing them up is expensive.

• Borehole depth — how deep the hole was drilled. Almost irrelevant to pump selection.
• Static (standing) water level — where water sits when the pump is off.
• Dynamic (pumping) water level — where water settles while pumping at the design flow. Static level plus drawdown.

The pump must be set below the dynamic water level with a margin, so that it stays submerged through seasonal decline and daily drawdown, and it must be set above the well screen and the bottom sediment, so it does not draw in sand or sit in silt. A common practice is to hang the pump a few metres above the bottom of the well and several metres below the lowest expected dynamic level — the exact figures come from the well's pumping test and driller's log, not from a rule of thumb.

Against that, check the pump's own limit. Maximum immersion is a specification, not a suggestion: it is the depth of water the motor and cable entry are rated to withstand. Across the Aquasolar range:

The 150 m immersion figure is the depth of submergence, and it is separate from the pump's maximum head. A pump rated for 260 m of head may only be immersed to 150 m — because the head figure describes how high it can lift water, while the immersion figure describes how much pressure the motor housing can take from outside. Deep boreholes routinely need the first without ever approaching the second, since the water level is usually far shallower than the drilled depth.

The 20 m immersion limit on the high-flow open-well series is a genuine selection filter: those units move 40–56 m³/h at 10–14 m of head, which is exactly what an open well or canal sump needs, but they must not be dropped into a deep borehole.

Pump length matters too

Overall length decides whether the pump can be manoeuvred into the well head and whether it clears any bend or step in the casing. Published lengths in the deep-well range run from 545 mm for the smallest 2″ screw up to 1,440 mm for the largest 6″ stainless unit. In shallow wells with limited water column, length also determines whether the whole pump can stay below the dynamic level.

Constraint 3 — Sand tolerance and water chemistry

Abrasion is the leading cause of premature failure in borehole pumps, and it is entirely predictable from a water sample. Two specifications matter.

Sand content. This is quoted as a mass fraction, and the difference between pump types is large:

A screw pump achieves this by moving water through a progressive cavity formed between a hard rotor and an elastomer stator — a rolling, low-shear action that passes fine sediment rather than accelerating it against metal vanes. The trade-off is flow: the screw series in this range deliver 1.0–3.6 m³/h, against up to 40 m³/h for the stainless impeller series. Screw pumps also excel at high head from low power, reaching 130–140 m on 750 W.

Water chemistry. The other half of the materials decision:

Also check the ambient operating envelope against your site. The published operating conditions for these series are a liquid temperature of 0–40 °C and pH 5–9, with sand limits as above. Water outside pH 5–9, or hot geothermal-influenced water, is a specification exception to raise explicitly with the manufacturer rather than to hope for.

A note on new boreholes

A freshly drilled or recently rehabilitated borehole produces far more sand than the same well will six months later. Development and flushing the well before the pump goes in — and installing above the screen — protects an impeller pump that would otherwise be destroyed in its first season by conditions that are temporary.

Constraint 4 — Duty point: flow at head, and pump type

Only now does hydraulic selection begin. You need the design flow at the total dynamic head — the calculation is set out in our step-by-step solar pump sizing guide — and you need to read catalogue figures correctly.

Headline head and flow are curve endpoints, not a duty point. A series quoted as "30–260 m head, 3–40 m³/h" does not contain a model that does 40 m³/h at 260 m. Within the range, high head comes with low flow and vice versa. Shortlist against the model-level rated figures, then confirm against the performance curve.

What each deep-well family is actually good at:

DC or AC/DC hybrid is the other axis. A DC pump runs from the array through an MPPT controller and fills a tank — simplest and best value where daytime pumping is enough. An AC/DC hybrid can also run from grid or generator power when solar is unavailable, which matters for domestic supply, clinics and guesthouses. Our note on DC versus AC/DC pumps covers the choice. Note that the AC/DC deep-well series start at 600 W, so very small duties are DC-only.

Constraint 5 — Cable sizing for the drop

The motor cable is part of the hydraulic system. Every volt dropped in the cable is head the pump does not produce, and undersized cable is the classic explanation for a pump that "underperforms" while every component tests good on the bench.

The published recommendation across these series is 2.5–4 mm² copper by model, sized for approximately a 50 m run, stepping up one cross-section per additional ~50 m of drop. Three things follow:

• Include the full run, not just the drop: array to controller plus controller to motor.
• Deep settings need a calculation, not the cable that came in the box. A 150 m drop is three 50 m steps beyond the reference length.
• Splices are a failure point. A submerged joint must be made with a proper heat-shrink or resin splice kit rated for permanent immersion. Tape is not a splice.

Fit a safety rope on any significant setting depth and secure the cable to the rising main at intervals so it cannot chafe against the casing. Retrieving a dropped pump from 100 m of borehole costs far more than the rope.

Controller, protection and monitoring

For deep-well installations the controller's protection suite matters more than on any other application, because the pump is inaccessible. The specification to look for — and what these series publish — is protection against overload, over-current, under-load, under-voltage, reverse connection, over-temperature and dry running, plus soft start. Dry-run (low-water) protection is the critical one: it is what saves the motor when the aquifer cannot keep up with the pump.

Two further points from the published data. Controllers up to 750 W are built into the pump assembly, while units from 1,100 W upwards use an external controller box that must be mounted out of the weather (the controller is rated IP66; the submersible motor is IP68). And the standard controller reports voltage, current, power, speed and error codes on a local LED display — remote monitoring is an optional add-on module, not standard, which is worth confirming if your project specification assumes telemetry.

Selection checklist

Before issuing an order, confirm you have all of these:

• Borehole inner diameter, plus depth and casing condition from the driller's log
• Static water level and drawdown at the design flow from a pumping test
• Sustainable well yield in m³/h
• Intended setting depth, checked against the pump's maximum immersion
• Water sample results: sand content, pH, salinity
• Daily volume required and total dynamic head
• Total cable run length, so cross-section can be calculated
• Pump OD in millimetres and overall length, in writing

If a supplier will quote a deep-well pump without asking for most of that list, treat the quotation as a price, not an engineering recommendation.

Next step

Browse the deep-well pump range or the 4″ borehole pump models, size your duty point with the free pump sizing calculator, or send your borehole diameter, water level, drawdown and daily volume for a specification-checked recommendation.

FAQ

What borehole diameter does a 4-inch solar pump need?

A 4″ solar submersible in this range has an actual outer diameter of 98 mm and a stated minimum borehole of 100 mm. That 2 mm margin is small, so allow for casing that may be out of plumb, scaled or partially collapsed, and for the space taken by the rising main coupling, cable and safety rope. Always work from the measured inner diameter of the casing rather than its nominal size, and ask the supplier for the pump OD in millimetres.

How deep can a solar submersible pump be installed?

Two separate limits apply. Maximum immersion is the depth of water the motor can withstand — 150 m for these deep-well series, and only 20 m for the high-flow open-well models. Maximum head is how high the pump can lift water, up to 260 m in this range. A borehole drilled to 200 m with water standing at 60 m needs a pump immersed well within 150 m while lifting from 60-plus metres, so both figures must be checked independently.

Can a solar pump handle sandy water?

Only within its rated sand content. Screw (progressive-cavity) pumps are rated to ≤0.25% sand and are the correct choice for silty or sandy wells, because the rolling cavity action passes fine sediment instead of accelerating it against metal vanes. Impeller pumps are rated to ≤0.05%, roughly 500 g/m³, and sand rapidly erodes their impellers and seals. Newly drilled boreholes should also be developed and flushed before an impeller pump is installed.

Should I choose a screw pump or an impeller pump for a deep borehole?

Choose a screw pump for high head at low flow, narrow casings and sandy water — models in this range reach 130–140 m on 750 W but deliver only 1.0–3.6 m³/h. Choose an impeller pump when you need more flow in clean water: the 4″ and 6″ stainless series reach up to 40 m³/h and up to 260 m of head. If your well is both deep and sandy but you need high flow, resolve the conflict with storage and longer pumping hours rather than by exceeding a sand rating.

What cable size does a deep-well solar pump need?

The published guidance for this range is 2.5–4 mm² copper depending on the model, sized for around a 50 m run, and stepping up one cross-section for every additional 50 m of drop. Count the whole circuit — array to controller and controller to motor — because voltage lost in cable is head the pump never produces. Any submerged joint must use a splice kit rated for permanent immersion, and the pump should carry a safety rope with the cable clipped to the rising main.

Do deep-well solar pumps need a storage tank?

In almost all cases yes, and it is the reason batteries are unnecessary. The pump fills an elevated or ground tank during daylight and the tank supplies demand around the clock, which stores energy as water at a far lower cost than an equivalent battery bank and with nothing to replace. Sizing storage at one to three days of demand also covers cloudy weather. Where water is genuinely needed at night on demand, an AC/DC hybrid pump with a grid or generator fallback is usually the better answer.