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What determines the size of a solar pumping system?
Four inputs drive the design[1]: the daily water demand in the month when demand is highest relative to sunshine, the total dynamic head (static water level, drawdown, pipe friction and delivery height or pressure), the solar resource in that design month, and the safe yield of the well or source. Getting any one of them wrong leads to a system that either runs short of water or over-pumps the well.
How is the system sized, step by step?
- Set the daily water demand for the design month and check it against the tested safe yield of the well.
- Calculate the total dynamic head: static water level, drawdown at the design flow, pipe friction and delivery height or pressure.
- Work out the daily hydraulic energy: volume in m3 × head in m × 0.002725 gives kWh per day.
- Divide by the pump and motor efficiency to get the daily electrical energy needed.
- Size the PV array: daily electrical energy ÷ (peak sun hours × system factor), then match the pump, controller and array.
- Store energy as water in a tank rather than in batteries wherever possible.
What does a worked example look like?
The factor 0.002725 kWh is the energy needed to lift one cubic metre of water by one metre, based on a water density of 1,000 kg/m3 and gravity of 9.81 m/s2. The example below uses illustrative assumptions, not design values for any particular site.
| Item | Value | Basis |
|---|---|---|
| Daily water demand | 100 m3 | Assumed |
| Total dynamic head | 150 m | Assumed |
| Daily hydraulic energy | 40.9 kWh | 100 × 150 × 0.002725 |
| Pump and motor efficiency | 50% | Assumed |
| Daily electrical energy | 81.8 kWh | 40.9 ÷ 0.50 |
| Peak sun hours, design month | 5.5 h | Assumed |
| System factor (heat, dust, wiring) | 0.80 | Assumed |
| PV array size | About 19 kWp | 81.8 ÷ (5.5 × 0.80) |
Why use water tanks instead of batteries?
A tank stores the day's pumping for use at night or on cloudy days at far lower cost than batteries, and it has no electronics to replace. Sizing the tank for one to three days of demand, depending on the use, usually gives better value than adding battery storage.
How do you protect the well and the aquifer?
Once installed, solar pumping costs almost nothing to run, which can encourage pumping beyond what the aquifer can sustain[2]. A pumping test before design, a flow limit set to the safe yield, dry-run protection on the pump and a water meter on the outlet keep the source productive.
Where independent support helps
An independent design review catches mismatched pumps, controllers and arrays before procurement, and a site check at handover confirms that the system delivers the design flow. Otibara has designed, reviewed and supervised solar pumping systems for municipalities, water utilities and NGOs across Lebanon.
Common questions
Does solar pumping work without batteries?
Yes. Most systems pump directly while the sun shines and store water in a tank. Batteries are added only where water is needed at specific times and a tank cannot provide it.
Can an existing diesel pump be converted to solar?
Often, by adding a solar pump inverter, if the motor is suitable for variable-speed operation. A hybrid with the generator kept as backup is common where demand is high. For sites that also need power for other loads, see the guide on solar PV feasibility studies in Lebanon.
Which standard covers solar pumping systems?
IEC 62253 sets requirements for the design qualification and performance measurement of photovoltaic pumping systems[3].
How long does a solar pumping system last?
PV modules typically carry long performance warranties, often 25 years. Pumps, motors and controllers need maintenance and usually replacement within the life of the array.