Size a deep-well pump for a solar pump inverter from three numbers together, not from horsepower alone: the motor power in kW, the total dynamic head, and the flow the well must deliver. Convert HP to kW, apply a 10–30% safety margin to the motor kW, then check that the inverter voltage, phase and output current match the motor and that the pump curve still delivers the required flow at that head. A pump chosen on HP alone can start, yet fail to lift water, trip on starting current, or run dry at the top of its curve.
The Three Sizing Inputs: Power, Head and Flow
Power decides the inverter rating. Head decides whether the pump can lift the water at all. Flow decides whether the daily water target is met. If any one of the three is missing, the other two cannot rescue the selection.
Start with the pump nameplate. Record rated power in kW or HP, rated voltage, phase, full-load current and, where the pump supplier provides it, the pump curve. Then build the total dynamic head: static water level, drawdown while pumping, the lift from the wellhead to the tank, and pipe friction at the design flow. For a full sizing method, see What Size Solar Pump Inverter Do I Need?, and for the deep-well problems that head and cable length create, see Solar Pump Inverter for Deep Well and Submersible Pumps.
HP to kW Conversion and the 10–30% Safety Margin
Use kW = HP × 0.746, the conversion used in How Do I Convert HP to kW for a Solar Pump Inverter?. The site sizing table rounds 1 HP to 0.75 kW, which is close enough for a first check, but keep the unrounded figure when you apply the margin. Then multiply motor kW by 1.10 for the minimum planning target and by 1.30 for the planning ceiling, and pick the next catalogue inverter size at or above the minimum target.
The margin is not generosity for its own sake. It covers starting current, which is several times full-load current for a short burst even with a soft ramp, voltage drop on a long motor cable, output derating in a hot pump house, and the extra current an older pump draws. Use the low end only when the nameplate current, cable length and installation conditions are all confirmed; use the high end for a deep borehole, a long motor cable, a hot installation or an older pump.
| Pump size | Motor power | Planning target with 10–30% margin | Next step to check |
|---|---|---|---|
| 1 HP | 0.75 kW | 0.83–0.98 kW | Next QB100-PV size at or above 0.83 kW |
| 2 HP | 1.5 kW | 1.65–1.95 kW | Next QB100-PV size at or above 1.65 kW |
| 3 HP | 2.2 kW | 2.42–2.86 kW | Next QB100-PV size at or above 2.42 kW |
| 5 HP | 3.7 kW | 4.07–4.81 kW | Next QB100-PV size at or above 4.07 kW |
| 7.5 HP | 5.5 kW | 6.05–7.15 kW | Next QB100-PV size at or above 6.05 kW |
| 10 HP | 7.5 kW | 8.25–9.75 kW | Next QB100-PV size at or above 8.25 kW |
| 20 HP | 15 kW | 16.5–19.5 kW | Next QB100-PV size at or above 16.5 kW |
How Head and Flow Derating Really Change the Power You Need
Head and flow are linked on the pump curve, not independent choices. At a higher head, the same pump delivers less flow; at a lower head, it delivers more. That is why a pump that looks correct on a datasheet can still miss a daily water target in a deep well: the static level falls under pumping, friction grows with flow, and the working point moves up the head axis and down the flow axis.
Do not answer a head shortfall by simply choosing a larger inverter. A larger inverter can feed more current, but it cannot make a pump produce head or flow beyond its curve, and it cannot lift a water column that the impeller cannot overcome at its rated speed. The correction is a pump stage, impeller or model change confirmed against the curve, followed by a re-check of motor kW, current, voltage and phase. The same rule applies in reverse: a pump with far more head than the well needs will run far from its best-efficiency point, waste array energy and risk short cycling against a tank or pressure switch.
Cable length matters in the same way. A deep well can mean 100–300 metres of motor cable between the wellhead drive and the submerged motor, as the deep-well guide notes. Voltage drop on that run raises the current needed to deliver the same shaft power, so a borderline inverter size that works on a bench test can trip or under-deliver once the cable is in the ground.
How to Size a Deep-Well Pump for a Solar Pump Inverter
- Record the nameplate. Note motor kW or HP, voltage, phase and full-load current. If the nameplate is worn, work from rated current and the pump supplier data, not from a guess.
- Convert HP to kW. Use kW = HP × 0.746 and keep the unrounded result for the margin calculation.
- Build the total dynamic head. Add static water level, drawdown while pumping, lift from wellhead to tank, and pipe friction at the design flow.
- Check the pump curve at that head. Confirm the pump delivers the required flow at the total dynamic head, not only at its best-efficiency point or at zero head.
- Apply the 10–30% margin. Multiply motor kW by 1.10 and by 1.30, choose the high end for a deep well, long cable, hot site or older pump, and pick the next catalogue inverter size at or above the minimum target.
- Match voltage, phase and current. A 380V three-phase borehole pump must not be paired with a 220V single-phase inverter, whatever the kW rating says. The inverter output current must cover the motor full-load current with the margin applied.
- Match the array window, then re-check the curve. Confirm the planned PV string voltage sits inside the inverter PV input class — DC 180–450V for 220V QB100-PV models and DC 250–900V for 400V models — and that the pump still meets flow at the head the well will actually present.
| Site condition | Margin to use | Why |
|---|---|---|
| Short cable, cool pump house, new pump, confirmed nameplate | 10–15% | Starting and ageing allowances are small and verified |
| Typical farm borehole, moderate cable run | Around 25%, or one size up | Covers starting current, heat and normal ageing |
| Deep well, long motor cable, hot site or older pump | 25–30% | Voltage drop, derating and higher running current stack together |
| Head or flow still unconfirmed | Do not finalise | Confirm the pump curve first; a margin cannot fix a wrong pump |
For the hardware that carries these ratings, compare the QB100-PV solar pump inverter (0.4–37kW), which covers 220V S2 and SS2 branches and 400V three-phase models, and the S80 solar pump inverter (0.75–200kW) where AC backup is also required. Send the nameplate, total dynamic head, required flow and cable length for a model confirmation.
Frequently Asked Questions
Can I size a deep-well pump from HP alone?
No. HP converts to motor kW, but it says nothing about head, flow or cable length. A correctly converted kW figure paired with the wrong pump curve still misses the water target or stalls against the head.
What happens if the well head is higher than I estimated?
Flow falls along the pump curve, daily water volume falls with it, and the pump may run for long hours without reaching the tank. Re-check the total dynamic head, then re-select the pump stage or model before changing the inverter.
Can I fix an undersized pump by adding more solar panels?
No. This is the trap in deep-well sizing. More panels can feed more watts to the inverter, but they cannot raise the inverter output current above its rating, cannot change the motor rating, and cannot make a pump produce head beyond its curve. Fix the pump and inverter ratings first, then size the array to the corrected system.
Do I always need the full 30% margin?
No. The 10–30% band is a planning range, not a single rule. Use the low end only when current, cable and conditions are confirmed; use the high end for deep wells, long cables, hot sites and older pumps. One size up, roughly 25%, is the common working choice in between.
Why does a long cable change the inverter size?
Voltage drop on a long motor cable means more current is needed to deliver the same shaft power. A size that is exact on paper can therefore trip at start-up or under-deliver in the well, which is why cable length belongs in the margin decision.
Not sure which factor limits your well? Send IDEEI your pump nameplate, total dynamic head, required flow and cable length. Contact IDEEI for a sizing recommendation.