Size a well pump
Computes total head from static lift AND pipe friction via Darcy-Weisbach. Over 60 m of 25 mm hose at 3 m³/h that adds 12 metres of friction — more than half the elevation difference. Sizing from the lift alone makes the pump too small.
The formulas behind the calculator
Every number above can be recomputed: the full calculation path, all assumptions and the data source with retrieval date — plus cross-validation against independent references. Disclosed, not claimed.
An estimate based on the stated assumptions. The final design must be checked by a qualified professional against the rules that apply where you are.
| Step | Formula | Value | Provenance |
|---|---|---|---|
| Flow velocity | Q / (pi * d^2 / 4) | 1.6977 m/s | exact |
| Reynolds number | v * d / nu | 42,272 | exact |
| Friction head | lambda * (L / d) * v^2 / (2 g) | 12.129 m | exact |
| Total head | H_static + h_friction + H_residual | 32.129 m | exact |
| Hydraulic power | rho * g * Q * H | 262.66 W | exact |
| Electrical power | P_hyd / (eta_pump * eta_motor) | 618.02 W | exact |
- Formula
h_f = λ · (L/d) · v²/(2g) · P_hyd = ρ · g · Q · H- Valid for
- Water at ambient temperature, circular cross-section, steady flow, full pipe.
- Not covered
- Local losses from bends, valves and check valves — in short, convoluted runs they can exceed the pipe friction. Also: cavitation, suction lift, and the specific pump curve.
Frequently asked questions
Why does my well pump deliver less than the elevation difference suggests it should?
Most likely pipe friction was left out of the sizing: over 60 m of 25 mm hose at 3 m³/h, roughly 12 metres of friction head are added — more than half the elevation difference. The calculator computes friction via Darcy-Weisbach, shows both components separately, and proposes the next commercial pipe diameter with its effect; friction head drops with the fifth power of the diameter.
What pipe roughness value should I enter?
Your pipe's datasheet value. The default of 0.15 mm corresponds to a used hose, not new PE pipe at 0.01 mm — in the reference case that is the difference between 12.1 and 8.1 metres of friction head. Entering the smoothest value undersizes the pump all over again.
Are bends, valves and check valves included in the head calculation?
No — local losses are not modelled, and in short, convoluted runs they can exceed the pipe friction itself; the calculator warns about this explicitly. Cavitation, suction lift and the specific pump curve are also outside the model; the calculation is valid for water at ambient temperature in a full pipe with steady flow.
Why is the thicker pipe almost always the better investment than the bigger pump?
Because pipe friction falls with the fifth power of the inner diameter: from 20 to 25 mm it drops to about a third, from 20 to 32 mm to a tenth. The chart computes the next standard diameter directly and shows the watts saved — which permanently translate into a smaller pump, less solar and less battery. Pipe is the cheapest efficiency in the whole system.
What does the flow velocity tell me?
It is the early warning: above about 2 m/s friction and wear rise noticeably, above 3 m/s it gets loud and water hammer looms. The design guide is 1–1.5 m/s for pressure lines. If your velocity is above that, the pipe is too thin for the flow — regardless of whether the pump still manages the friction.
Why does the calculator use Darcy-Weisbach instead of blanket values?
Because friction depends nonlinearly on diameter, flow, roughness and flow regime: the friction factor comes from the Reynolds number (laminar/turbulent) and relative roughness. Blanket “add 10%” rules are off by multiples for long thin runs — exactly the typical garden-irrigation-from-well case.
How much water does a solar pump deliver per day?
The sun decides, not the datasheet. For a pump with 2 m³/h flow and 234 W draw on 800 W of modules in the Berlin region the calculator finds 15.2 m³ a day in the best month and 1.6 m³ in the worst — about 3,216 m³ over the year. The pump only runs while the modules deliver more than it draws, and on dull winter days that threshold is barely reached.
How much array power does my well pump need?
Considerably more than the draw suggests. The same 234 W pump on 400 W still delivers 8.3 m³ a day in June — but nothing at all in December, because the array never crosses the starting threshold in any hour. With 800 W, 1.6 m³ remain in December. The rule of thumb 'array power equals twice pump power' holds for summer; year-round operation needs more.
Does a wider pipe beat a stronger pump?
With solar operation, doubly so. Going from 25 to 32 mm inner diameter cuts the friction head and with it the draw from 234 to 205 W. That means not only less energy per cubic metre but a lower starting threshold: the pump runs 1.3 instead of 0.9 hours a day in December and delivers 3,482 instead of 3,216 m³ over the year — 8 % more water from the same system.
Why does the pump fall short in winter?
Because a directly coupled solar pump has a threshold: below its draw it does not run at all rather than pumping more slowly. In December an 800 W array in central Europe provides enough in only a few hours — the calculator puts it at 0.9 run hours a day. Anyone needing water in winter cannot avoid storage: either water in an elevated tank or electricity in a battery.
Should I store water or electricity?
As a rule, water. An elevated tank costs a fraction of a battery of the same energy, does not age and needs no electronics. The calculator supplies the figure: if the worst month delivers 1.6 m³ a day and you need 3 m³, you have to carry stock over from the good months — not make the pump bigger. A battery only pays when pumping has to happen at fixed times.