Short answerA pump curve plots flow against head; your operating point is where the curve crosses your total dynamic head, not the maximum flow or maximum head on the listing. Choose a pump whose curve passes your duty point in its middle third.
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Key takeaways
- Maximum flow is at zero head and maximum head is at zero flow: neither is your operating point.
- Solar pumps have a family of curves, one per power level.
- Choose a pump whose curve passes your duty point in its middle third.
Flow against head
A pump curve plots flow (gpm) against total dynamic head (ft). At zero head the pump delivers its maximum flow. As head rises, flow falls, until at the maximum head (the shut-off head) it delivers nothing at all. The headline figures on a listing are the two ends of that line.
VEVOR's own listing for its 24V 277W pump makes the point: maximum flow 9.25 gpm, maximum head 272 ft, but a rated point of 5.28 gpm at 84 ft. At 150 ft it will deliver less again.
Solar curves have a third axis
A mains-powered pump has one curve. A solar pump has a family of them, one for each power input or irradiance level, because the panels deliver less in the morning, in cloud and in winter. Good manufacturers publish curves at several wattages or a table of daily output at several heads and sun-hour totals. Use the line that matches your array size and design-month sun.
Pick a pump that is comfortable at your duty point
- Your duty point (flow at your TDH) should sit in the middle third of the curve, not at either end.
- Near shut-off head, a small loss of power or a drop in water level stops flow completely.
- Near maximum flow, the pump is lifting almost nothing and wasting most of its power.
- Different pump types have different curve shapes. Centrifugal pumps have steep curves that collapse at high head. Diaphragm and helical rotor (positive displacement) pumps hold flow far better as head rises, which is why they dominate deep solar wells.
If there is no curve
Many budget listings give only the two maximums and perhaps one rated point. Treat the rated point as the most useful figure, assume flow falls steeply above it, and ask the seller for the curve before buying for a lift above about half the maximum head. Our product comparison lists every figure each merchant publishes.
Three DC well pumps side by side on the merchant's own figures, plus panels, controllers and batteries.