Solar pump duty point calculator
A pump is specified by two numbers together: the flow it must deliver at a given total dynamic head. This works out both, from your well and your water demand.
Take the result to any supplier and ask them to mark that exact point on the pump's flow-versus-head curve, or to supply a test report at that duty point. A catalogue max flow and a catalogue max head are two different points on the same curve and do not prove the one you need.
The well
Rest level below the wellhead, pump off.
How much further the level falls while pumping.
The delivery
Site-dependent. Use your own irradiance data.
10 m ≈ 1 bar. Drip and sprinkler lines need this.
The pipe run
Riser plus the run to the tank or field.
The bore, not the nominal outside size. Same nominal size in a different pressure class has a different bore.
Bends, valves, non-return valve.
Your duty point
The pump must deliver
10.00 m³/h
at a total dynamic head of
69.3 m
= 166.67 L/min at 69.3 m
| Static water level | 40.0 m |
| Drawdown while pumping | 8.0 m |
| Discharge elevation above wellhead | 6.0 m |
| Pipe friction | 4.8 m |
| Fittings allowance (10%) | 0.5 m |
| Required pressure at the outlet | 10.0 m |
| Total dynamic head | 69.3 m |
We check the duty point against the pump curve before quoting.
How this is calculated
Total dynamic head is the sum of five things, not just the depth of the well:
TDH = static water level + drawdown + discharge elevation + friction loss + required outlet pressure
Friction loss uses the Hazen-Williams equation:
hf = 10.67 · L · Q1.852 / (C1.852 · D4.87)
L in m, Q in m³/s, D in m (internal), hf in m
Method and assumptions. Read this before you put the number in a tender.
The Hazen-Williams equation and the roughness coefficients above are published, widely used values for water at ordinary ambient temperature. They are a starting assumption, not data for your specific pipe: confirm C against the pipe manufacturer's own figures, and confirm the internal diameter against the actual pressure class you are buying.
The fittings allowance is a percentage shortcut. On a run with many bends, valves or a long riser, replace it with an itemised equivalent-length calculation.
Usable pumping hours is a site input, not a constant. Use measured or modelled irradiance for the site rather than a national average.
This page works out a requirement. It does not pick a pump. That comes off the performance curve of the model being offered.
Longer explanations: how total dynamic head is built up and how to read a pump curve.
What to ask any supplier
Three sentences you can paste into an email. We answer all three in our own quotations without being asked, and you should expect the same from anyone else.
1. Please confirm the flow at 69.3 m total dynamic head, not the maximum flow.
2. Please send the flow-versus-head curve for the exact model offered, with that point marked.
3. If you cannot mark that point on a curve, please send a test report taken at that duty point.
Send us the number
Once you have the duty point, send it over and we come back with a pump, its curve with your point marked, and a priced bill of materials.
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