Convert HP to kW by multiplying the pump motor HP by 0.746, then choose a solar pump inverter at least 10–30% above the resulting motor kW. In practice, that usually means picking the next inverter size up from the motor rating — never an inverter rated exactly at, or below, the motor power. If the motor nameplate already gives kW, use that kW figure directly; the nameplate always wins over a converted HP figure.
For example, a 5 HP pump is about 3.7 kW, so the planning target is 4.1–4.8 kW before you check the available catalogue sizes, voltage and phase. The full chart below works the same arithmetic for the common pump sizes, and the sizing guide linked at the end maps those sizes to the IDEEI QB100-PV range.
How to Convert HP to kW in Three Steps
- Read the motor nameplate. Record HP, kW if shown, voltage, phase and rated current. A worn nameplate is a reason to check rated current and pump data first, not a reason to guess.
- Convert HP to kW. Use kW = HP × 0.746. The existing IDEEI 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.
- Apply the 10–30% safety margin, then pick the next size up. Multiply motor kW by 1.10 for the minimum target and by 1.30 for the planning ceiling, then choose a catalogue inverter at or above the minimum target. Use the upper end for a deep borehole, a long motor cable, a hot installation or an older pump; use the lower end only when the nameplate current, cable length and installation conditions are all confirmed.
- Check voltage and phase before ordering. As the IDEEI sizing guide notes, 220V single-phase systems dominate below roughly 5.5 kW, while larger three-phase borehole pumps normally use 400V. A correct kW number on the wrong voltage or phase is still the wrong inverter.
HP to kW to Inverter Size: Quick Chart
| Pump size | Motor power | Inverter target at +10% | Inverter target at +30% |
|---|---|---|---|
| 1 HP | 0.75 kW | 0.83 kW | 0.98 kW |
| 2 HP | 1.49 kW | 1.64 kW | 1.94 kW |
| 3 HP | 2.24 kW | 2.46 kW | 2.91 kW |
| 5 HP | 3.73 kW | 4.10 kW | 4.85 kW |
| 7.5 HP | 5.60 kW | 6.15 kW | 7.27 kW |
| 10 HP | 7.46 kW | 8.21 kW | 9.70 kW |
These figures are arithmetic from 1 HP = 0.746 kW, rounded to two decimals. They are a planning target, not a substitute for checking rated current, starting behaviour, cable length and the inverter manual.
Why the 10–30% Margin Matters
A pump motor is not a fixed resistor. The IDEEI sizing guide gives three reasons installers rarely match inverter and motor one-to-one: inverter output capacity falls as ambient temperature rises, deep boreholes mean long motor cables and voltage drop, and an older pump can draw more current than its nameplate suggests. The margin buys headroom against all three at once. It is not wasted capacity: one size up is good practice, while several sizes up is not, because a heavily oversized inverter costs more and can be harder to match to the pump and array.
What Happens If the Inverter Is Too Small
| Risk | What you may see | What to check first |
|---|---|---|
| Repeated trips or shutdowns under load | The pump stops when demand or temperature rises | Motor rated current against inverter rated output |
| Heat derating | Output falls on hot days, even with sun available | Shade, airflow clearance and the manual ambient limit |
| Slow or weak pumping | Lower speed and flow than the pump curve promises | Cable length, voltage drop and array output at the same time |
| No room for an older or deeper duty | A system that worked when new struggles later | Whether the original size left any margin at all |
If any of these symptoms appear, do not simply restart and hope: re-check the nameplate, the cable run and the installation conditions, and ask for a sizing review before the drive is run repeatedly at its limit.
For the wider sizing picture, see What Size Solar Pump Inverter Do I Need? A Practical Sizing Guide with HP/kW Table and, for a deep-well example, How to Select a 7.5kW Solar Borehole Pump Inverter for a Farm in Kenya. For product ranges, compare the QB100-PV S2 (220V) and QB100-PV 1.5–37 kW (400V) pages and confirm the exact model against your nameplate. To request a sizing check, send the pump nameplate data — motor power, voltage, phase, well depth and daily water target — through the contact page.
Frequently Asked Questions
What is the formula to convert HP to kW?
Multiply HP by 0.746. So 5 HP × 0.746 = 3.73 kW, and 10 HP × 0.746 = 7.46 kW. To go the other way, divide kW by 0.746.
If my pump is 5 HP, can I buy a 3.7 kW inverter because the labels match?
No — that is the trap. 3.7 kW is the motor power, not the inverter target. Add the 10–30% margin first, giving 4.10–4.85 kW, then pick a catalogue size at or above that target and check rated current, voltage and phase.
Is 1 HP exactly 0.75 kW?
No. 1 HP is 0.746 kW; 0.75 kW is a rounded figure used in the IDEEI sizing table for a quick check. The difference is small for one pump, but use the nameplate kW whenever it is printed on the motor.
Should I always use the 30% end of the margin?
Not always. Use the upper end for deep boreholes, long cables, hot sites or older pumps, and the lower end only when the nameplate, cable and installation are all confirmed. What you should not do is use 0% margin.
Does a bigger solar array fix an undersized inverter?
No. More array power cannot raise an inverter above its rated output. Size the inverter from the motor first, then size the array to feed it — the sizing guide notes an array of about 1.3 to 1.6 times motor power as a working rule for solar pumping.
What if my pump size is not in the chart?
Apply the same three steps: HP × 0.746, then × 1.10 and × 1.30, then pick the next catalogue size up and check current, voltage and phase against the product page and manual.