On farms far from the grid, or where electricity is expensive, solar irrigation systems run a submersible or surface pump directly from PV modules. A well-sized system delivers water steadily through the sunny hours. A poorly sized one starts late in the morning, fails to reach the required flow even at noon, or stresses the drive with excessive voltage. In this article we walk through the inputs, the role of the pump drive, how the array is sized, and when a system is not a good fit, using a worked example.
System components
- PV array: produces DC power. Batteries are usually not used; a water tank often takes the place of energy storage.
- Pump drive (solar pump inverter / VFD): converts DC from the array into variable-frequency AC for the pump motor.
- Pump and motor: a submersible or surface pump.
- Cables and protection: the DC run, the AC run between drive and motor, fuses, a DC disconnector and surge protection.
- Water tank or reservoir (recommended): stores water pumped during sunny hours for use when needed.
The inputs
Flow requirement
Daily water demand depends on the crop, the irrigated area, the method (drip, sprinkler, flood) and the season. Divide the daily demand by the number of hours of effective solar pumping to get the hourly flow. For example, if you need 120 m³ a day and expect 6 hours of effective pumping, the required average flow is 20 m³/h.
Well depth and total head
The total head the pump must overcome (total dynamic head) is the sum of:
- the depth to the dynamic water level during pumping (not the static level; the level drops as water is drawn),
- the elevation difference from the wellhead to the tank or irrigation point,
- friction losses in pipes and fittings,
- the outlet pressure required by the drip or sprinkler system.
Pump power
Pump motor power is usually given in horsepower (HP) or kW; 1 HP ≈ 0.746 kW. The hydraulic power needed to lift the water is:
P hydraulic (W) = ρ × g × Q × H, where ρ is water density (≈1,000 kg/m³), g is gravitational acceleration (9.81 m/s²), Q is flow (m³/s) and H is total head (m).
Shaft power is hydraulic power divided by pump efficiency. Always check the manufacturer's pump curve (flow vs head) when selecting a pump.
Daily irrigation hours
Sunny hours vary with the season. Irradiance is low in the morning and evening, so the pump either runs slowly or cannot lift water at all. Keep the "effective pumping hours per day" assumption realistic.
Cable length
With a submersible pump, the motor sits at the bottom of the well, so the cable between drive and motor is as long as the well depth plus the surface distance. Voltage drop on a long cable makes the motor draw more current and run hotter.
What the pump drive does
Unlike an ordinary frequency converter, a solar pump drive performs MPPT (maximum power point tracking): it finds the highest power the array can deliver at the current irradiance and sets motor speed accordingly. Speed drops as irradiance falls and rises as it increases.
- Soft start: starts the motor at low frequency, limiting inrush current.
- Protection: stops the pump on dry running, overcurrent, over- or undervoltage and similar conditions (features vary by model).
- Input voltage window: every drive has an MPPT operating range and a maximum DC input voltage that must never be exceeded. The array is designed around that window.
One piece of physics matters a lot here: for centrifugal pumps, flow varies in proportion to speed while head varies with the square of speed. So when speed drops a little at low irradiance, the head the pump can deliver drops noticeably. In deep wells this means that below a certain speed the pump cannot bring any water to the surface.
Worked example
All figures below are example values. A real project must use the pump curve, the module and drive datasheets and site measurements.
1. Hydraulic power and pump size
- Flow: 20 m³/h = 20 ÷ 3,600 ≈ 0.00556 m³/s
- Total head: 120 m
- Hydraulic power: 1,000 × 9.81 × (20 ÷ 3,600) × 120 = 6,540 W
- With a pump efficiency of 60%, shaft power is 6,540 ÷ 0.60 = 10,900 W ≈ 10.9 kW
- The next standard motor size up, 15 HP (15 × 0.746 ≈ 11.19 kW), is selected.
2. Array power
To get enough power in the morning and evening and to cover temperature and cable losses, array power is chosen above motor power. Drive and pump manufacturers usually recommend a factor; this example uses 1.3.
- Target array power: 11.19 × 1.3 ≈ 14.55 kWp
- With 550 Wp modules: 14,547 ÷ 550 ≈ 26.4 → at least 27 modules
3. Series string check
Example module values: Voc = 49.9 V, Vmp = 41.9 V, Voc temperature coefficient −0.27%/°C, and roughly −0.35%/°C for Vmp. Example drive: MPPT range 450–750 V DC, maximum DC input 800 V. Design temperatures: coldest ambient −10 °C, hottest cell temperature 70 °C.
- Module Voc at −10 °C: 49.9 × (1 + 0.0027 × 35) ≈ 54.62 V
- Module Vmp at 70 °C cell temperature: 41.9 × (1 − 0.0035 × 45) ≈ 35.30 V
| Modules in series | Cold Voc (V) | Hot Vmp (V) | STC Vmp (V) | Result |
|---|---|---|---|---|
| 13 | 710 | 459 | 545 | Marginal: hot Vmp very close to the MPPT minimum |
| 14 | 765 | 494 | 587 | Suitable |
| 15 | 819 | 530 | 629 | Not suitable: exceeds the 800 V limit |
Choice: 14 in series × 2 in parallel = 28 modules, giving 28 × 550 = 15.4 kWp. That is 15.4 ÷ 11.19 ≈ 1.38 times motor power, slightly above the 1.3 target. Finally, check that the combined current of the two parallel strings does not exceed the drive's maximum DC input current.
4. Motor current and cable
Approximate current for a three-phase motor: I = P ÷ (√3 × U × cosφ × η). Example: 11,190 W, 400 V, cosφ = 0.85 and motor efficiency 0.88 give I ≈ 21.6 A. In a real project, use the rated current on the motor nameplate.
Voltage drop (ignoring reactance) is approximately ΔU = √3 × I × L × cosφ ÷ (κ × A), with κ ≈ 56 m/(Ω·mm²) for copper. Example: 100 m of well plus 50 m on the surface gives 150 m of cable; with a 10 mm² conductor, ΔU ≈ 1.732 × 21.6 × 150 × 0.85 ÷ (56 × 10) ≈ 8.5 V, or about 2.1% of 400 V. Check the relevant standards and the pump manufacturer's recommendation for the acceptable limit.
Fuse and cable considerations
- Fuses, disconnectors and cables on the DC side must be DC-rated and selected for the array's maximum voltage. Do not use AC switchgear on the DC run.
- As the number of parallel strings grows, a faulty string can receive reverse current from the others. Whether string fuses are needed, and their rating, depends on the maximum series fuse rating in the module datasheet and the relevant standards.
- Follow the drive manufacturer's recommended fuse or circuit breaker ratings for input and output protection.
- Submersible pump cable must be water-resistant, and joints must be insulated with suitable materials.
- Open fields carry a high risk of lightning and surges; do not neglect earthing and surge protection.
When solar irrigation is not a good fit
- Irrigation must happen at night or at fixed times and no water tank can be installed.
- The well yield is lower than the pump flow: the pump will run the well dry; flow must be limited to the well's yield.
- The head is very high: at low irradiance the pump cannot lift water, and effective pumping hours shrink considerably.
- The installation area is shaded or there is not enough space for the array.
- The existing motor and drive are incompatible: motor voltage and phase count must match the drive output.
- Uninterrupted water is needed during cloudy periods and a hybrid (grid- or generator-backed) solution is not on the table.
Checklist
- Are daily water demand and effective pumping hours defined?
- Is total head calculated from dynamic water level, elevation and friction losses?
- Is the pump selected against the manufacturer's curve?
- Is the series module count within the drive window for cold Voc and hot Vmp?
- Have voltage drop and fuse ratings been checked?
Solar irrigation sizing with PVAGE
In PVAGE's agricultural energy systems module, you enter pump power (HP), well depth, daily irrigation hours and cable length (line loss), then choose a module. The result shows the module count and layout, motor current, recommended fuse and a "suitable / not suitable" assessment. The factors used in the calculation are shown on screen, and the result can be exported as a PDF.
In the proposal module, "agricultural irrigation" can be selected as the system type. For other off-grid applications, see our article on off-grid system sizing.