Technical Sizing Guide: How to Calculate the Perfect Submersible Well Pump

1. The Fundamental Concepts

Before taking out the calculator, we must define the two physical quantities that determine the "Working Point" of the pump.

A. The Range (Q)

It is the amount of water the pump must move per unit of time. It is usually measured in liters per minute (l/min) or cubic meters per hour (m³/h) .

• Household needs: An average family needs around 20-30 l/min peak.
• Irrigation: Depends on the number of sprinklers. A static sprinkler uses about 5-10 l/min; a dynamic (pop-up) sprinkler can reach 15-20 l/min.

B. Total Manometric Head (H)

It is the pump's ability to lift water by overcoming gravity and friction. It is expressed in meters of water column (mca) or in bars (1 bar ≈ 10 mca).

2. The Formula for Calculating Prevalence

Calculating the prevalence is the phase where most errors are made. The complete technical formula is:

Where:

• H _geo (Geodetic Height): It is the vertical distance between the dynamic water level in the well (not the bottom of the well!) and the highest point of use.
• P _res (Residual Pressure): This is the pressure we want at the tap outlet. For a standard domestic or irrigation system, approximately 3 bar is needed. Since 1 bar = 10 meters, we'll consider 30 meters .
• P _c (Head Loss): It is the energy lost by the water due to friction as it flows through pipes and passes through valves, bends and filters.

3. Practical Calculation Example

Let's get to work with a real case you may have to deal with.

The Scenario:

• Two-storey villa with garden.
• The well is 60 meters deep.
• The water, when the pump is on (dynamic level), stabilizes at -35 meters above the ground.
• The highest user is the shower on the first floor, +5 meters above the ground.
• We want to water the garden and serve the house at the same time.
• Total length of the pipe from the well to the house: 100 meters .
• Tube diameter: 1" (32mm) polyethylene.

Step 1: Calculate the required flow rate (Q)

Let's say we want to use a shower (15 l/min) and two sprinklers (15 l/min each) at the same time.
Q _tot = 15 + 15 + 15 = 45 l/min
This is our project scope.

Step 2: Calculating the Geodetic Height Difference (H _geo )

H _geo = 35m (well) + 5m (house) = 40 meters

Step 3: Estimating the Pressure Drops (P _c )

Water flowing at 45 l/min through a 32mm pipe generates friction. Consulting the technical tables (see below), we discover that a 1" pipe with 45 l/min has a loss of approximately 8.2 meters for every 100 meters of pipe .
Let's add a safety margin for curves and valves:
P _c ≈ 9 meters

4. Quick Reference Table: Pressure Losses in Pipes

To facilitate the calculation of the P _c value (Head Loss), the AR StoreShop technical team has developed this simplified table.

The values indicate the pressure loss expressed in metres (mca) for every 100 metres of straight polyethylene (PE) pipe .

As you can see in the highlighted line: with 45 l/min and a 1" hose, we lose 8.2m. If we had used a 1 1/4" hose, we would have lost only 2.5m!

Final Step: Calculating the Total Prevalence (H _tot )

We want 3 bar of useful pressure (P _res = 30 m) at the tap. Applying the formula:

H _tot = 40m (H _geo ) + 30m (P _res ) + 9m (P _c )
H _total = 79 meters

5. How to Read the Characteristic Curve

Now we have our license plate data:

• Required flow rate: 45 l/min
• Required head: 79 meters

Beware of a common mistake: Don't buy a pump with "Max. Head: 80 m." That data refers to zero flow! You should look in the AR StoreShop catalog for a pump whose characteristic curve matches our data.