How to size a solar pump system: the method

Start from the water the farm needs

A pump is sized to a job: how much water is needed in a day, and how high it has to be lifted. Ask the farmer for the daily requirement, the depth of the water source and the height to the delivery point or tank.

The total head is not just the depth. It also includes the height to the outlet and the friction in the pipe, so measure or estimate each part and write down which one you used.

Choose the pump and the drive

Use the pump's performance curve or datasheet to find a motor that gives the needed flow at the total head. A solar pump runs from the panels through a variable frequency drive with no battery in between, so the drive's datasheet decides what the array must provide.

Check the drive's input window, the range of voltage it accepts, and the pump's rating. If the pump is quoted under a ministry scheme such as PM-KUSUM, the module line also has to meet the DCR condition, which the DCR guide explains.

Size the array to the drive

The array has to supply at least what the drive and pump need, plus an allowance for losses from heat, dust, wiring and the drive. The drive's datasheet usually states the array it accepts, and it is the reference to follow.

To find the number of modules, divide the array capacity by the rating of one module and round up. Then check that the modules wired in series give a voltage inside the drive's window in both hot and cool weather, and that the layout fits the available land or structure.

State the assumptions

A pump quotation is only as reliable as its assumptions, so write them down.

• The daily water need and the total head used.
• The make, model and rating of the pump, the drive and each module.
• The sunlight and the losses you allowed for, described as assumptions.
• Whether storage, such as a tank, is part of the system, because the pump runs only when there is sun.

What the method cannot promise

Water output rises and falls with the sun through the day and through the seasons. A sizing method can show that the system suits the stated need, but it cannot guarantee a daily volume, so do not write one into the quotation.

Step by step

1. 1

   Fix the water need

   Record the daily requirement, the depth of the source and the height to the outlet.

2. 2

   Work out the total head

   Add the lift and the pipe friction, and note how you estimated each.

3. 3

   Choose the pump and drive

   Pick the motor from the datasheet curve, then read the drive's requirement.

4. 4

   Size the array

   Divide the array capacity by the module rating, round up, and check the voltage window.

5. 5

   Write the assumptions

   State the need, the head, the equipment and the losses allowed, and promise no volume.

Common mistakes

• Counting panels without the drive

  The drive's input window and power decide what the array must provide, so a panel count alone can leave the system out of range.

• Using only the depth as the head

  The height to the outlet and the pipe friction add to the head, and ignoring them undersizes the pump.

• Promising a daily volume

  Water output follows the sun, so a promised volume cannot be guaranteed.

• Forgetting storage

  The pump runs only when there is sun, so a farm that needs water at other hours needs a tank.

What to check before you send

• The daily water need and the total head are written down, with how each was found
• The pump, drive and module lines name their make, model and rating
• The array was sized to the drive's datasheet and the voltage window was checked
• The losses allowed are stated as assumptions
• The module line meets the DCR condition if the pump is quoted under a ministry scheme
• The quotation promises no daily water volume

Sources

• MNRE office memorandum on the DCR verification portal, read 10 Oct 2026