Most solar pump mistakes happen before the pump even arrives. The two numbers that decide everything are total dynamic head (TDH) — how much height and friction the pump must push against — and flow rate — how many liters per hour it must deliver at that head. Buy against those two numbers, not against the well depth or the brand name on the box, and the system will work for years.
Why total dynamic head is not the well depth
The water level you pump from is the static water level — and it drops further while the pump runs (drawdown). On top of that, the pump lifts water above ground to the tank, and friction inside the pipe adds hidden "extra meters" that many buyers forget. A 30 m well with 10 m drawdown, a 5 m tank, and 150 m of narrow pipe can demand well over 70 m of TDH. That is exactly what this calculator adds up for you, using the Hazen–Williams friction formula for the pipe length and diameter you enter.
From daily water need to hourly flow
Divide your daily liters by the sun hours you have. A farm needing 5,000 L/day with 6 peak sun hours must find a pump that gives at least ~840 L/h at the calculated TDH. Pump makers publish curves in m³/h against meters — so compare the result the calculator gives you with the manufacturer's curve at your TDH, and check the real flow is at or above your requirement.
Matching the solar array to the pump
Hydraulic power (ρ·g·Q·H) tells you the physics; the motor and drive efficiency tell you the reality. Divide hydraulic power by the combined efficiency (55–70% for a modern VFD setup) and add ~15% margin for heat, clouds and voltage losses. The result is the array size in watts. Undersized arrays are the number one reason pumps "don't work" — they run, but only a few hours a day, delivering far less than promised.
A worked example
A 30 m static level, 10 m drawdown, 5 m delivery height and 150 m of 25 mm pipe at 840 L/h gives a friction of roughly 15 m — TDH ≈ 60 m. The hydraulic power at that point is about 1.4 kW; at 60% drive efficiency the realistic array is around 2.6–3 kW of panels, roughly six 450 W modules. That is a typical installation for a small farm plot of a few hectares.
Practical tips from the field
In dry seasons the water level drops — always enter the worst-case drawdown, not the level you measured today. Use a tank instead of batteries: storing water is far cheaper than storing energy, and the pump can run all day into the tank. And never reduce pipe diameter to save money: friction rises sharply in smaller pipes and silently eats your daily output.
Frequently asked questions
Why do I need more head than the depth of the well?
Because the pump must also push water through pipes. Friction inside the pipe acts like extra height, and any delivery height above ground adds to it. A shallow well with a long horizontal run can need more head than a deep well with a short pipe. The calculator adds pipe friction automatically once you enter your pipe length and diameter.
How do I convert my daily water need into the flow rate the pump must deliver?
Divide your daily water need (liters or gallons) by the number of pump hours you are willing to run during daylight. For example, 6,000 liters over 6 sun hours needs a pump that delivers at least 1,000 liters per hour at the calculated total dynamic head. The calculator does this conversion for you.
Why is total dynamic head more important than well depth when buying a solar pump?
Because pumps are rated against total dynamic head (TDH), not well depth. TDH is the static water level plus drawdown, plus delivery height, plus pipe friction. Buying a pump rated for the well depth alone often results in zero or very poor flow once the water level drops or pipes are added.
How many panels do I need to run the pump?
It depends on the pump's power at your TDH and flow, and your local sun hours. A pump using 1,500 W of hydraulic and motor power with a VFD in 5 sun hours typically needs a 2,500–3,000 W array to run through clouds and heat losses. The calculator estimates this from your inputs.
Can the same pump work with different heads?
Yes — but the flow changes. Pump curves show flow falling as head rises. At low head you get high flow; at high head the flow drops, sometimes to nothing. That is why the calculator shows you both the flow you will get at your TDH and the daily water volume over your sun hours, so you can check the match before buying.
What happens if I undersize the panels for the pump?
The pump still runs but starts late in the morning, stops early in the afternoon, and produces a fraction of its rated water. In practice you lose 30–50% of daily output for a small saving in panel cost — which is why pumps should always be matched to the array, not the other way around.