A deep well is the hardest workplace a solar pump can draw. The pump and motor sit tens or hundreds of metres underground, at the end of a long cable, lifting water against a head that never lets up — and nobody is standing next to it when something goes wrong. Choosing a solar pump inverter for deep well and submersible pump duty is therefore less about the headline kilowatts and more about three things: whether the drive can hold torque against a high, constant head, whether it protects a motor it cannot see, and whether it keeps working unattended for years. This guide works through those questions with the ratings used across the IDEEI deep-well range, from small boreholes to 280kW irrigation wells.
Why Deep Wells Are a Different Pumping Problem
A surface pump works against friction and a modest lift. A submersible pump in a deep well starts every run already loaded: the water column above it is a constant weight, the motor is sealed in a steel tube surrounded by water, and cooling depends entirely on flow past the motor body. Two consequences follow. First, starting current matters — a direct-on-line start slams the column upward, stresses the cable and can sand-lock or surge the well; a drive that ramps frequency up gently avoids all three. Second, failure is invisible. A pump running dry at 120 metres makes no sound at the surface, which is why the protection set — dry-run detection, water-level sensing and current monitoring — is part of the inverter specification, not an accessory.
Head Rules Everything: Sizing for Depth
Size the system from the total dynamic head: static water level, drawdown while pumping, the lift from wellhead to tank, and pipe friction at your design flow. That head-flow point chooses the pump; the pump motor chooses the inverter. Apply the standing rules: pick an inverter about 25% above the motor’s rated power so it is not running at its limit all day, and size the PV array at roughly 1.3–1.6 times the motor power so the pump reaches useful speed early and holds it through thin cloud. Do not be tempted to oversize the pump itself for a well whose sustainable yield is modest — a pump that out-draws the aquifer spends its life cycling on dry-run protection, and the well, not the electronics, becomes the limit.
The Long Cable Run: Voltage Drop and Starting Torque
Between the inverter at the wellhead and the motor down the hole there may be 100–300 metres of cable. At 220V that distance eats voltage: the motor sees a sagging supply exactly when it needs starting torque, and a marginal design trips or stalls. This is the practical reason deep wells standardise on 380V three-phase submersible motors — higher voltage means lower current for the same power, less drop over the run and a thinner, cheaper cable. Size the cable for the full run at the motor’s rated current, keep the drop within a few percent, and remember the inverter’s soft ramp is also a cable-stress control: current climbs smoothly instead of spiking at several times nominal on every start.
One Platform from Borehole to Production Well: LX200
Deep wells span a huge power range — a 0.75kW household borehole and a 75kW farm production well are the same problem at different scales. The IDEEI LX200 solar pump inverter platform (0.4–500kW) is built for exactly that spread: it covers both 220V and 380V motor classes, and its MPPT-based solar control varies pump speed with the available PV power, so the pump starts early, tracks the sun through the day and slows gracefully instead of tripping when a cloud crosses. Its pump-oriented protection set is designed for unattended running — the drive supervises the motor it cannot see and shuts down safely on dry-run or abnormal current rather than letting a sealed submersible motor cook in an empty well. Where the site has a grid or generator connection, AC backup arrangements are available depending on the model and wiring, which suits wells that must also run at night.
Large Wells and High Flows: The Iron-Shell QB100-PV
At the top of the range — production wells for irrigation schemes, estates and small towns — the motors are big three-phase machines and the inverter becomes a cabinet-class installation. The QB100-PV iron-shell range (45–280kW, 400V three-phase) covers that territory in eleven steps: 45, 55, 75, 90, 110, 132, 160, 185, 200, 250 and 280kW. These are systems designed on paper before they are ordered: the well’s test-pumping data, the cable run and the array layout all feed the selection, and the inverter step is matched to the motor with the usual headroom rather than rounded down to a price. If your project sits in this band, send the pump curve with your enquiry — at these powers, a one-step sizing error costs more than the engineering time that prevents it.
Start-Up Behaviour in Weak Light
Deep-well pumps have a hidden threshold: below a certain speed the impeller cannot lift the column to the surface, and the pump churns without delivering water. A well-matched solar drive handles this honestly — it waits until the array can sustain a speed above the delivery threshold, then runs productively, rather than spinning uselessly from dawn. In practice this means a correctly sized array (that 1.3–1.6 factor again) does more for daily water volume than a nominally larger inverter: the water you count is the water delivered above the threshold, hour by hour, and MPPT speed control is what converts morning and evening light into extra tank volume instead of heat in the motor.
How to Check a Deep-Well System Before Ordering
Work through this checklist in order. If any step fails, change the pump, the cable or the inverter before ordering — a deep-well mistake is expensive to correct after the pump is down the hole. For the detailed sizing method behind steps 2 and 3, see How Do I Size a Deep-Well Pump for a Solar Pump Inverter?
- Measure total dynamic head. Add static water level, drawdown while pumping, lift from wellhead to tank, and pipe friction at design flow. Size from this total, not from depth alone.
- Match the pump to the head-flow point. Confirm the chosen pump delivers the required flow at that head, and that its motor power and voltage class suit the well.
- Size the inverter with headroom. Allow inverter headroom above the motor rated power — the linked sizing guide applies a 10–30% margin table — and size the array at roughly 1.3–1.6 times motor power so the pump reaches useful speed early and holds it through thin cloud.
- Size the cable for the full run. Calculate voltage drop over the whole wellhead-to-motor length at motor rated current, and prefer 380V three-phase motors for long runs, as this article explains.
- Confirm dry-run and level protection. Check that dry-run detection, water-level sensing and current monitoring are included for a motor nobody can watch.
- Check weak-light start-up. Confirm the array and drive will lift the water column in morning and evening light, not only at midday.
Frequently Asked Questions
Can a solar pump inverter run a submersible pump 150 metres down?
Yes — depth itself is not the limit; head, cable drop and motor class are. With a 380V three-phase motor, a correctly sized cable and an inverter matched to the motor with headroom, 150-metre installations are routine. Send the well depth, water level and motor nameplate for an exact check.
Do I need a special inverter for a submersible pump?
You need a pump-duty inverter rather than a general-purpose one: MPPT solar input, controlled ramping for the water column, and dry-run and water-level protection for a motor nobody can watch. The LX200 platform is specified around exactly that duty.
What protects the pump if the well runs dry?
The drive watches motor current and the level sensors: a submersible pump drawing air pulls abnormal current, and the inverter stops it and waits for the well to recover instead of letting the motor overheat in a dry casing.
Is a bigger pump always better for a deep well?
No — this is the trap. A pump that out-draws the aquifer spends its life cycling on dry-run protection, and the well, not the electronics, becomes the limit, as this article explains. Size the pump to the well sustainable yield and the head-flow point, not to the largest motor available.
What well data do I need before sizing?
Well depth, static water level, drawdown while pumping, the lift from wellhead to tank, pipe details, the pump model and your daily water target. With those figures the full method in How Do I Size a Deep-Well Pump for a Solar Pump Inverter? can be applied, including its HP-to-kW conversion and 10–30% margin table.
Planning a deep-well or submersible solar pumping project? Send IDEEI your well depth, static and dynamic water levels, pump model and daily water target — we will match the inverter and array to the well, not just to the motor. Contact IDEEI for deep-well sizing.