Choosing a plunger pump: capacity, lift, and performance before converting to solar power

Last updated: October 18, 2026

A submersible pump is selected based on three key factors: total lift in meters (from the water level inside the well to the discharge point, including pipe losses), the required flow rate in cubic meters per hour, and the motor power needed to efficiently achieve this. If the pump is unsuitable from the outset, no solar system, however large, can fix it; therefore, pump selection should precede calculations for panels and inverters.

Why choose the pump before calculating the solar system?

A solar-powered submersible pump system is designed to supply a specific motor with a specific power output. If the pump is overpowered, it will draw excessive force from the solar panels; if it's underpowered, it will operate at a reduced capacity or shut down when solar radiation drops in the morning and evening. Many farm wells had their pumps installed years ago, but the water level has since dropped, causing the pumps to operate beyond their intended range. This manifests as increased motor current and reduced water flow.

That's why we always start with the question: How much water do you need, from what depth, and to where? (If you're new to the idea, start with a guide.) Solar power for operating agricultural submersible pumpsNext comes the calculation of the panels and inverter, which we explained in the article. Solar submersible pump system calculation based on horsepower and well depth.

What is total lift and how is it calculated?

Total Dynamic Head is the equivalent vertical distance that the pump must overcome. It is not equal to the depth of the well, but rather consists of several parts:

1. Still water level: The depth of the water surface inside the well before operation.
2. Drop during pumping: When the pump is running, the water level around it drops, depending on the well's productivity. The figure that matters is the dynamic level during pumping.
3. Height above ground level: If the water is poured into a raised tank or basin at a higher level than the wellhead.
4. Friction losses: Every meter of pipe and every elbow and valve adds resistance, and it increases as the pipe diameter decreases or the flow rate increases.
5. Required operating pressure: If the pump is feeding a drip irrigation network or direct sprinklers, the required pressure must be added, converted to meters (approximately 1 bar equals 10 meters).

The depth of the pump installation itself does not directly affect the lifting, but it is important for another reason: the pump must remain submerged below the dynamic level with a safety margin so that it does not draw in air or run dry.

How do you know the water level in your well?

The most accurate method is to measure the water level with a water level meter before starting the pump and after approximately one hour of continuous pumping. If this is unavailable, the well drilling log or previous pump maintenance records can provide a preliminary estimate, but an actual measurement is preferable because groundwater levels change over time.

How do you determine the required action?

The flow rate is the amount of water per hour. Start with the daily needs of the farm: the type of crop, the area, and the irrigation method, or the number of livestock, or the needs of the house and rest area. Then divide the daily need by the hours of pumping.

Here's the key difference with solar energy: a diesel-powered or grid-tied pump might run for 12 hours or more, while with solar panels, efficient pumping is concentrated during sunny daylight hours, with lower flow in the early morning and late afternoon. Therefore, you might need a higher-flow pump to collect the same amount of water in fewer hours, or a tank to store water for irrigation, which is the subject of another article. Water tanks and irrigation network with solar submersible pump system.

And pay attention to an important limit: the required action must not exceed the well's own production capacity. Extracting more water than the well can replenish will lead to a drop in the water level, requiring the pump to run dry, or forcing sand to be drawn out.

How do you read a pump performance curve?

Every submersible pump has a manufacturer's curve that relates lift to flow: the higher the lift, the lower the flow. Plot your working point (total lift and required flow) on the curve:

• If the point falls near the middle of the curve, this is usually the area of highest efficiency, and it is the best choice.
• If it falls at the top of the curve (high lift and very low flow), the pump is operating near its limit and any drop in the well level or decrease in radiation will stop the water flow.
• If it falls on the right side (high conductivity and low lift), the motor current may increase and wear may increase.

In solar systems, the motor's frequency changes throughout the day, causing the pump to shift to lower speeds in the morning and evening. Therefore, we choose a pump that maintains a clear lift margin above your actual lift, allowing you to start pumping water earlier in the day.

Illustrative example: From the well to the engine power

This is just an example for illustration purposes; the figures are approximate.

Item	The value in the example	note
Still water level	80 m	Pre-operation size
Drop during pumping	15 m	Dynamic level 95 m
Tank height above ground	5 m	Tank on a base
Friction losses	Approximately 8 m	According to the diameter and length of the pipes
Total lifting	Approximately 108 m	We round it up to 110 m
daily needs	120 m³	Irrigation and livestock
Effective solar pumping hours	6–7 hours	It varies according to the season
Required action	18–20 m³/h	120 ÷ 6.5 approximately

The hydraulic power required to lift 20 m³/h to a height of 110 m is approximately 6 kW. Since the combined efficiency of the pump and motor is less than 100% (often in the range of 50–70% depending on size and model), the required motor power becomes approximately 9–12 kW, or roughly 12.5 to 15 hp. We then review the available pump specifications and select the model whose efficiency range falls within the 110 m and 20 m³ points. Finally, we design the panels based on the power of this motor, as shown in the diagram. Solar power system to operate a 15 hp submersible pump.

Other specifications not to overlook when buying a diver

Well diameter and pump diameter

Submersible pumps come in common diameters such as 4, 6, and 8 inches. The pump body and its attached cable must be inserted into the well with sufficient clearance, and water must circulate around the motor to cool it. A narrow well with a large pump means difficulty in lowering the pump and overheating of the motor.

Motor voltage and type

Solar submersible inverters primarily work with three-phase motors (380V for medium and large sizes; some smaller inverters output 220V three-phase for smaller motors). Single-phase motors with a starting capacitor are generally not compatible with frequency control, so if your current pump is single-phase, you will likely need to replace either the motor or the pump during the conversion.

Raw material and water quality

Salty or acidic water requires a suitable stainless steel pump. Sand-laden water accelerates wear on the wellbore, so it's preferable to choose pumps that can handle a higher sand content and address the underlying cause at the well itself, if possible.

cable and tube

The deeper the installation, the longer the cable and the greater the voltage drop; therefore, the cable cross-section must be calculated based on the current and length. The riser pipe must also withstand the pressure of the water column and the weight of the pump.

Is my current solar-powered pump suitable?

Before deciding on replacement, gather the following information and send it to the engineer:

• Motor data plate: Power, voltage, current, number of phases.
• The depth of the pump installation and the depth of the water level, if known.
• The diameter of the well and the diameter of the riser pipe.
• The quantity currently being pumped out and the duration of its operation.
• Pump age and condition of last maintenance.

If the three-phase pump is in good working order and its operating point is suitable, it can often be powered by a solar submersible inverter without modification. If it is old or has low efficiency, it is more cost-effective to replace it with a conversion kit rather than designing a complete system for a worn-out pump. For a comparison of operating costs versus continuing with diesel, see [link/reference]. Solar pump or diesel generator?.

Frequently Asked Questions