How to read a solar pump performance curve
A pump curve is one line on a chart, and almost everything a buyer needs is somewhere on it. The part specific to solar is that there is never only one line: a variable-speed drive moves the pump across a whole family of curves as the sun goes up and down.
One line, two ends
Where it meets the vertical axis is shut-off head. Where it meets the horizontal axis is free delivery. Neither is your site.
Your point is a crossing
The system curve rises as flow grows. Where it crosses the pump line is what the installation actually does.
Solar moves the line
Speed follows irradiance, so the pump slides between curves through the day. The catalogue line is the noon one.
What is on the chart
| Vertical axis: total head | In metres, sometimes in bar or feet. It is what the pump is working against, not how deep the well is. |
| Horizontal axis: flow | In m³/h or L/min. Catalogues from Chinese factories usually print both scales on the same axis, one above the other. |
| The line itself | One line per model, or per impeller stage count. Where it meets the vertical axis is shut-off head; where it meets the horizontal axis is free delivery. |
| Several lines on one chart | A family. Each line is a different model in the same series, normally differing in stage count and motor power, and the legend names them. |
To read your own number: find your total dynamic head on the vertical axis, move horizontally until you hit the pump line, then drop straight down. Where you land on the flow axis is what that pump gives you. If your head is above where the line starts, the pump cannot reach your site at any flow.
The second line: your system
The pump curve describes the pump. It says nothing about your pipework. The system curve is the other half, and it is drawn from your own numbers.
| Static head | The part that never changes with flow: lift from water level to discharge point. A flat line across the chart. |
| Friction head | Grows with flow, roughly as flow to the power 1.85. A curve that bends upward. |
| System curve | The two added together. It starts at the static head on the vertical axis and climbs to the right. |
| Duty point | Where the system curve crosses the pump curve. That intersection is what the pump will actually do on your site, and it is the only point that matters. |
This is why changing pipe size changes the pump you need. A narrower pipe makes the system curve steeper, so it crosses the pump line further left, at less flow. Nothing about the pump changed.
What makes a solar curve different
A mains pump runs at one speed all day and has one curve. A solar pump is driven at whatever frequency the array can support, so it runs at full speed for a few hours around noon and slower either side. Three relationships govern what that does, and they are worth knowing because they are not intuitive:
| If speed changes by | Flow changes by | Head changes by | Power changes by |
|---|---|---|---|
| a factor n | the same factor n | n squared | n cubed |
| down to 70% | down to 70% | down to 49% | down to 34% |
| down to 50% | down to 50% | down to 25% | down to 13% |
The practical consequence
Head falls with the square of speed. A system with a high static head has a cut-off: below a certain speed the pump cannot reach the discharge point at all and delivers nothing, however long the sun is up. A low-head system keeps producing water at low speed instead.
So two installations with the same array and the same pump can have very different daily volumes purely because of static head. This is the part a single catalogue curve will never tell you, and it is why we ask for the head before quoting rather than after.
The efficiency line, and why it costs you modules
Larger catalogues print an efficiency curve alongside the head curve, peaking somewhere in the middle of the flow range. Running far from that peak wastes shaft power and wears the pump faster.
On a mains installation that shows up as an electricity bill. On a solar installation it shows up as modules: the array is usually the single most expensive line on the bill of materials, and a pump running at poor efficiency needs a bigger one for the same water. It is worth a conversation before the pump is fixed, not after.
What to ask for, and what to check on it
| Model number on the chart | A series chart with six lines is not proof of anything. The model being offered has to be identified on the sheet. |
| Your point marked | Flow and head written next to a marked point, not implied by the shape of the line. |
| The speed it was taken at | Full speed, normally 50 or 60 Hz. If the curve is at a reduced speed without saying so, everything else is wrong. |
| A test report if you are near the edge | If the duty point sits close to the left or right end of the curve, a report taken at that point is worth asking for. |
We put the model number, the curve and the marked duty point in our own quotations without being asked.
Have a curve you want checked?
Send it over with your head and flow and we will mark the point and tell you plainly whether it works. Same if you have an offer from someone else and want a second read.
Pump curve FAQ
How do you read a pump performance curve?
Head is on the vertical axis and flow on the horizontal. Find your total dynamic head on the vertical axis, move across to the pump line, then drop down to the flow axis. That flow is what the pump delivers at your head. Anything printed as a headline maximum sits at one end of that line, not where you are.
What is the duty point on a pump curve?
It is where the system curve crosses the pump curve. The system curve starts at your static head and rises with flow because friction grows; the pump curve falls with flow. They cross at exactly one point, and that intersection is the flow and head the installation will actually run at.
Why do solar pump curves look different from mains pump curves?
A mains pump runs at one speed, so it has one curve. A solar pump is driven at a frequency that follows the irradiance, so it moves between a whole family of curves through the day. The catalogue curve is the full-speed one, which is the best case around solar noon rather than a typical hour.
What happens to flow when the pump slows down?
Flow falls roughly in proportion to speed, and head falls roughly with the square of speed. So at 70% speed a pump gives around 70% of the flow but only about half the head. That is why a system with high static head stops producing water long before the sun goes down, while a low-head system keeps trickling.
What does the efficiency curve on a pump chart tell me?
It shows where the pump converts the most shaft power into water. Running far to the left or right of the best efficiency point wastes energy and increases wear. On a solar system it also matters because the array is the most expensive part: a pump running at poor efficiency needs more modules for the same water.
Should I ask for a curve for the series or for the exact model?
For the exact model. A series chart shows several lines and it is easy for the wrong one to be implied. Ask for the model number written on the chart with your duty point marked, or a test report taken at that point.
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