How Many Solar Panels Does the Pump Really Need?

Solar pump inverter PV array sizing should begin with daily water demand and total dynamic head, not with a panel count copied from another project. Two sites can use the same motor and still need different arrays because the water level, pipe loss, solar resource, seasonal demand and pumping window are different.

This design notebook follows one illustrative quotation from water to watts. It is written for solar-pump contractors, pump manufacturers and distributors using WLD280 in tropical or high-solar off-grid markets. The calculation is a screening method, not a final electrical design.

Table of Contents

1. The quotation inputs
2. Convert water and head into energy
3. Add real pump and system losses
4. Use design-month solar hours
5. Select array power and verify instantaneous duty
6. Check string voltage and current
7. Build the final quotation sheet

Solar Pump Inverter PV Array Sizing: The Quotation Case

A grower needs 45 cubic metres of water per day. The measured pumping water level, storage-tank elevation, pipe friction and outlet requirement produce a total dynamic head of 52 m. The pump will fill a tank during daylight, so irrigation does not need to follow sunshine minute by minute.

Before calculating, write down what is known and what is still assumed:

The values below are intentionally visible so a contractor can replace them. They are not measured BEDFORD customer results.

Step 1: Calculate Hydraulic Energy

For water, a practical daily estimate is:

Hydraulic energy (kWh/day) = 0.002725 x daily volume (m3) x total head (m)

For the example:

0.002725 x 45 x 52 = 6.38 kWh/day

This is the ideal energy delivered to the water. It does not include pump, motor, controller, cable or PV losses.

The World Bank Solar Water Pumping Handbook starts system design with water demand, pumping time and total dynamic head. That order prevents a common sales error: selecting modules before the hydraulic job is defined.

Step 2: Account for Pump and Motor Efficiency

If the combined pump and motor efficiency at the selected duty point is 55%:

Electrical energy at the motor = 6.38 / 0.55 = 11.60 kWh/day

Efficiency must come from the pump curve at the actual duty, not from a best-case catalog headline. A pump operating far from its best-efficiency region may require more input and wear faster.

Check seasonal water level as well. A deep well that draws down by 12 m in the dry season changes the head and therefore the energy requirement. Design around the critical water-demand month, not the easiest month.

Step 3: Use the Design-Month Solar Resource

Peak sun hours convert the day’s varying irradiance into an equivalent number of hours at 1 kW/m2. They are not the number of bright daylight hours.

The NREL PVWatts Calculator can estimate monthly PV energy from location, array size, orientation and losses. Use the month that combines important water demand with a defensible solar resource. FAO irrigation guidance likewise emphasizes the critical demand period and local solar radiation.

For this example, assume 4.8 design-month peak sun hours per day and a 0.75 delivery factor:

Initial PV size = 11.60 / (4.8 x 0.75) = 3.22 kWp

The delivery factor is not a universal BEDFORD setting. It is a transparent allowance for module temperature, soiling, mismatch, wiring, controller conversion and other system losses. NREL’s default losses are useful for screening, but the actual project assumptions should be documented.

Step 4: Do Not Stop at Daily Energy

An array that delivers enough energy across a day can still be too small to run the selected pump at the required instantaneous head. Check the pump shaft and motor input power at the duty point.

Suppose the selected pump needs 3.0 kW electrical input at the required operating point. The preliminary 3.22 kWp array leaves little margin for hot modules, dust or normal irradiance variation. The design may need a larger array, a longer pumping window, a different pump or more storage.

This is why there is no honest universal rule such as “1.3 times motor kW” for every site. A multiplier can be a supplier screening value only after the hydraulic duty, climate and controller limits are known.

Step 5: Choose Water Storage Before Adding Batteries

Daily irrigation demand may peak in the early morning or evening while PV output peaks around midday. A tank or reservoir decouples pumping time from use time.

Set storage from the operating plan:

• required evening and early-morning volume;
• one or more low-sun days accepted by the project;
• source-well recovery rate;
• tank high and low levels;
• overflow route and dry-run protection;
• required emergency backup.

Storage is not free energy. It adds elevation head and may increase the calculation. Include the maximum operating tank level in total dynamic head.

Step 6: Verify the WLD280 Voltage Class and String

The BEDFORD WLD280 Solar Pump Inverter supports PV DC input, AC input arrangements, three-phase pump motors and well/reservoir level signals. The current product page lists:

The exact model must pass motor voltage and rated-current checks. Then use the selected module datasheet to verify cold-corrected string Voc, hot operating Vmp, parallel-string current and protective-device ratings.

Do not add series modules merely to increase kWp. The BEDFORD solar pump inverter DC voltage notebook shows why the same total wattage can produce safe or unsafe string designs.

Step 7: Test the Daily Water Result

Commissioning should record more than “pump runs.” Log:

1. irradiance condition and module temperature;
2. array voltage and current;
3. controller output frequency and motor current;
4. pumping water level and tank level;
5. flow and total head;
6. time to deliver the daily water target;
7. weak-light sleep, restart, dry-source and full-tank responses.

Use the BEDFORD solar pump inverter low-sunlight guide if morning cycling or cloud-related restarts appear. More panels are only one possible answer.

Reusable Contractor Worksheet

Solar pump inverter PV array sizing becomes defensible when a buyer can trace every panel back to water demand, head, efficiency, sunshine and electrical limits. WLD280 then becomes the controller inside a designed pumping system, not a shortcut around the design.