Calculate total head, not borehole depth
Total head is the vertical lift plus friction losses plus discharge pressure. Borehole depth on its own is only part of the first term, because the pump sits below the water level and the water level draws down while pumping.
Formula: total head = static water level + drawdown + friction loss + discharge head. Friction loss is typically 5–10% of pipe length for a well-chosen pipe diameter, and much more if the pipe is undersized.
| Component | Value |
|---|---|
| Borehole depth | 45 m |
| Static water level below ground | 18 m |
| Drawdown while pumping | 6 m |
| Friction loss, 60 m pipe at 12 m³/h | 4 m |
| Discharge head above ground (tank inlet) | 3 m |
| Total head | 31 m |
Match flow rate to crop area
Required flow depends on the irrigated area, the crop water requirement and how many hours per day you can pump. More pumping hours means a smaller, cheaper pump for the same daily volume.
| Crop | Daily requirement | Flow needed over 8 h pumping | Flow needed over 4 h pumping |
|---|---|---|---|
| Maize | 40–55 m³/ha | 5–7 m³/h | 10–14 m³/h |
| Vegetables | 45–65 m³/ha | 6–8 m³/h | 11–16 m³/h |
| Rice (paddy) | 70–100 m³/ha | 9–12 m³/h | 18–25 m³/h |
| Citrus and fruit trees | 30–45 m³/ha | 4–6 m³/h | 8–11 m³/h |
| Greenhouse drip | 25–40 m³/ha | 3–5 m³/h | 6–10 m³/h |
Size the array with a margin for real-world losses
A common rule is 1.3 to 1.4 times the pump rated power for shallow installations, rising with head. Deep boreholes need more because the pump has to overcome both lift and friction, and because hot afternoons reduce panel output exactly when demand peaks.
| Total head | Minimum array | Recommended array | Expected daily volume |
|---|---|---|---|
| 20 m | 1.8 kWp | 2.0 kWp | 70–90 m³ |
| 40 m | 2.0 kWp | 2.4 kWp | 55–70 m³ |
| 60 m | 2.6 kWp | 3.0 kWp | 35–50 m³ |
| 80 m | 3.2 kWp | 3.6 kWp | 25–38 m³ |
| 100 m | 3.8 kWp | 4.4 kWp | 18–28 m³ |
Compare running cost honestly
Diesel and solar pumps cost about the same to buy for an equivalent duty, so the decision is really about running cost and reliability. Fuel logistics and generator maintenance dominate the total cost of ownership for diesel.
| Item | Diesel pump | Solar pump |
|---|---|---|
| Equipment | Baseline | Baseline, plus battery if 24 h demand |
| Maintenance over 5 years | Engine service, injectors, filters | Impeller and controller only |
| Energy cost over 5 years | Fuel and delivery to site | Zero |
| Downtime risk | High where fuel delivery is unreliable | Low, weather-dependent volume only |
Frequently asked questions
01Can a solar pump run from mains as well as panels?
02How do I protect the pump from running dry?
03Do I need a battery for a solar pump?
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