How Many HP Does a Submersible Well Pump Need?

Choosing the correct horsepower (HP) for a submersible well pump is one of the most important steps in pump selection.

Many buyers start by asking:

“How many horsepower pump do I need for my well?”

However, horsepower cannot be selected only from well depth or pipe size. A pump with a larger motor is not always better. The correct motor size depends on the complete hydraulic requirement, including flow rate, Total Dynamic Head, pump efficiency, water level, and operating conditions.

A professional pump supplier should first calculate the required duty point, then select a pump and motor that can operate efficiently at that condition.

Before selecting motor power, the required flow should be confirmed. You can read our guide on how to calculate the required flow rate for a submersible well pump and how to calculate Total Dynamic Head for a well pump.

How Many HP Does a Submersible Well Pump Need? Quick Answer

A submersible well pump cannot be assigned a specific horsepower from well depth alone. The required motor output depends mainly on the required flow, Total Dynamic Head, pump efficiency and the absorbed power shown on the pump performance curve.
For clean water, the preliminary calculation is:
Hydraulic Power, Ph (kW) = Flow, Q (m³/h) × Head, H (m) ÷ 367
Pump Shaft Power, P2 (kW) = Hydraulic Power ÷ Pump Efficiency
Estimated Electrical Input Power, P1 (kW) = Pump Shaft Power ÷ Motor Efficiency
Pump efficiency and motor efficiency must be entered as decimals. For example, 75% should be entered as 0.75.
The motor output rating should be selected from the required pump shaft power—not from the electrical input power. The final motor must be able to carry the absorbed pump power throughout the permitted operating range.
For example, a pump delivering 80 m³/h at 100 m TDH with 75% pump efficiency requires approximately 29.1 kW, or 39 HP, of shaft power. A standard motor rating must then be confirmed from the selected pump’s performance and power curves.

What Does HP Mean in a Submersible Well Pump?

HP means horsepower and is a measurement of power. In a submersible well pump system, the motor provides the energy needed to move water against resistance.

SLAPK’s published industrial submersible well pump range currently covers approximately 7.5–410 kW, or 10–550 HP. Available motor ratings depend on pump diameter, hydraulic model, number of stages, voltage, frequency and project configuration.

However, the same horsepower motor can be matched with different pump hydraulics. A 10 HP motor does not automatically produce a specific flow or head.

The final performance depends on:

① Pump design
② Number of stages
③ Impeller size
④ Efficiency
⑤ Operating point

This is why buyers should compare pump performance curves instead of comparing horsepower alone.

Why Horsepower Cannot Be Selected by Well Depth Alone

One of the most common mistakes is choosing motor power based only on the depth of the well.
For example:
A 200 m deep well does not automatically require a 30 HP pump.
The actual requirement depends on:
① Dynamic water level
② Required flow
③ Discharge elevation
④ Pressure requirement
⑤ Pipeline friction loss
A 200 m well with a dynamic water level of 80 m may require much less power than a 120 m well where the water level falls to 100 m during operation.
The correct method is to calculate the complete duty point first, then select the motor.
If the operating water level is not known, first confirm the difference between static and dynamic water level and record the dynamic level at the intended pumping rate.

How to Calculate Submersible Well Pump Horsepower

Pump power should be calculated in stages because hydraulic power, pump shaft power and electrical input power are not the same value.
For water, when flow is expressed in cubic metres per hour and head in metres:
Hydraulic Power, Ph (kW) = Q × H ÷ 367
Where:

• Q = flow rate in m³/h
• H = Total Dynamic Head in metres
• 367 = metric conversion constant for water

The approximate power required at the pump shaft is:
Pump Shaft Power, P2 (kW) = Hydraulic Power ÷ Pump Efficiency
The approximate electrical input power is:
Electrical Input Power, P1 (kW) = Pump Shaft Power ÷ Motor Efficiency
To convert between kilowatts and horsepower:
HP = kW × 1.341
kW = HP × 0.746
These calculations provide a preliminary power requirement. The final motor rating must be checked against the manufacturer’s performance curve and power curve at the actual duty point.

Worked Example: Calculate the Required HP

Assume the required operating conditions are:

• Required flow: 80 m³/h
• Total Dynamic Head: 100 m
• Pump efficiency at the duty point: 75%
• Motor efficiency: 90%

Step 1: Calculate hydraulic power
80 × 100 ÷ 367 = 21.8 kW
Step 2: Calculate required pump shaft power
21.8 ÷ 0.75 = 29.1 kW
Step 3: Convert shaft power to horsepower
29.1 × 1.341 = 39.0 HP
Step 4: Estimate electrical input power
29.1 ÷ 0.90 = 32.3 kW
The calculated pump shaft requirement is approximately 29.1 kW, or 39 HP. The estimated electrical input is approximately 32.3 kW because the motor is not 100% efficient.
These values have different purposes. The 29.1 kW shaft requirement is used as the basis for selecting the motor output rating, while the 32.3 kW electrical input is used to estimate the power drawn from the electrical supply.
A 30 kW motor may appear to be the nearest standard rating, but it should not be confirmed from this calculation alone. The manufacturer must check the absorbed-power curve across the intended operating range, together with the permitted motor load, voltage conditions, starting method, cooling conditions and approved design margin.

Pump Horsepower Formula for GPM and Feet

When flow is expressed in US gallons per minute and head in feet:
Water Horsepower = Flow (GPM) × Head (ft) ÷ 3,960
Brake Horsepower = Water Horsepower ÷ Pump Efficiency
Specific gravity must also be included when the pumped liquid is significantly different from clean water.
Always use one complete unit system throughout the calculation. Do not mix m³/h with feet or GPM with metres.

Common kW and HP Conversions

These are approximate conversions, not automatic pump selections.

Factors That Determine Submersible Well Pump Horsepower

The Relationship Between Flow, Head, and Motor Power

Flow and head determine the hydraulic workload of the pump.
A simple relationship is:
Higher Flow + Higher Head = Higher Motor Power Requirement
For example:
Project A:

• Flow: 50 m³/h
• TDH: 80 m

Project B:

• Flow: 50 m³/h
• TDH: 160 m

Although the flow is the same, Project B requires approximately twice the hydraulic energy before considering efficiency differences.
This is why pump suppliers should never recommend motor power only from flow or only from well depth.
The complete duty point should always be checked on the submersible well pump performance curve.

How Pump Efficiency Affects Required HP

Two pumps with the same flow and head may require different motor sizes because their efficiencies are different.
For example:
Pump A:

• Efficiency: 75%

Pump B:

• Efficiency: 60%

Pump B requires more input power to produce the same hydraulic output.
For projects operating many hours per day, efficiency has a major effect on operating cost. A slightly more efficient pump may save significant electricity expenses over its lifetime.
When comparing quotations from different suppliers, do not compare only:

• Pump price
• Motor HP
• Flow rate

Also compare:

① Efficiency at the duty point
② Energy consumption
③ Recommended operating range
④ Motor quality

Submersible Well Pump HP Selection by Application

How to Select HP for Irrigation Well Pumps

Irrigation systems often require pumps with continuous operation and stable performance.
The required HP depends on:
① Irrigation flow requirement
② Sprinkler pressure
③ Field elevation difference
④ Pipeline length
⑤ Filter losses
For example:
An irrigation system requires:

• Flow: 100 m³/h
• TDH: 120 m

The supplier should select a pump that can deliver this duty point efficiently.
Installing a higher-horsepower motor on the same pump at the same speed does not automatically increase flow or pressure. Flow and head are determined mainly by the pump hydraulic design, operating speed and system resistance.
If the complete pump is oversized for the irrigation system, it may cause:

• Excessive flow or pressure
• Well over-pumping
• Higher equipment and energy costs
• Operation outside the recommended range
• Additional throttling and control losses
• Increased wear

The objective is to select a pump that delivers the required irrigation duty point efficiently, then match the motor to the absorbed power of that pump.

How to Select HP for Deep Well Water Supply

Deep well water supply projects often involve larger vertical lifts and longer operating hours.
The motor selection should consider:
① Dynamic water level
② Pump installation depth
③ Required flow
④ Storage tank height
⑤ Pipeline losses
⑥ Daily operating time
For example, a municipal or industrial project running 20 hours per day should pay close attention to efficiency because energy cost becomes a major part of the total ownership cost.
A professional submersible well pump manufacturer should evaluate the complete system instead of simply recommending a high-power motor.

Problems Caused by Incorrect Pump and Motor Sizing

Oversized Pump vs. Oversized Motor

An oversized pump and an oversized motor are not the same problem.
A larger motor connected to the same hydraulic pump at the same operating speed does not normally make the pump produce more flow or head. It provides more available motor capacity, but it may increase purchase cost and operate at an unnecessarily low load.
An oversized hydraulic pump can create more serious system problems, including:

• Excessive flow or pressure
• Well over-pumping
• Operation away from the best-efficiency region
• Additional valve throttling
• Higher energy consumption
• Increased hydraulic wear

If the calculated shaft requirement is 15 HP, a 25 HP motor should not be selected automatically as a safety measure. The final motor rating should be based on the pump’s absorbed-power curve, permitted operating range, voltage, starting method, cooling conditions and manufacturer requirements.

Undersized Motor Problems

An undersized motor may be unable to carry the absorbed power required by the selected pump.
Possible results include:

• Motor overload
• Excessive current
• Motor overheating
• Frequent protective trips
• Failure during high-flow operation
• Reduced motor service life
• Unreliable water delivery

An undersized motor does not automatically change the hydraulic curve of the pump. The system may initially approach the required flow and head, but the motor can overload, slow down or trip because it cannot sustain the required shaft power.
The motor should therefore be checked against the maximum absorbed power within the intended operating range, not only against the power calculated at one duty point.

Motor Service Factor and Safety Margin

Do not assume that every submersible motor has the same service factor or that the service factor can be used for continuous intentional overloading.
Motor ratings and allowable loading may differ according to the motor design, applicable standard, voltage, frequency, cooling conditions and manufacturer.
Before applying a design margin, confirm:

1. Rated motor output
2. Maximum permitted current
3. Manufacturer-approved loading
4. Voltage tolerance
5. Starting method
6. Required cooling flow
7. Ambient water temperature
8. Permitted number of starts
9. Continuous or intermittent duty

Use only the loading allowance stated by the motor manufacturer. A service factor should not be used to compensate for an incorrectly selected pump.

How to Compare Different Pump Horsepower Quotations

Different manufacturers may recommend different HP motors for the same project.

This can happen because they use different:

① Pump efficiencies
② Impeller designs
③ Number of stages
④ Safety margins
⑤ Operating assumptions

When comparing quotations, check:

① Required flow
② Required TDH
③ Motor HP
④ Efficiency
⑤ Duty-point location
⑥ Rated current
⑦ Pump materials
⑧ Warranty

A 20 HP pump from one manufacturer is not automatically equivalent to a 20 HP pump from another manufacturer.

The performance curve and actual duty point are more important than the motor number.

What Information Does a Supplier Need Before Selecting HP?

A professional supplier needs complete project information before recommending motor power.

Prepare:

① Required flow rate
② Total Dynamic Head
③ Static water level
④ Dynamic water level
⑤ Well diameter
⑥ Installation depth
⑦ Pipe size and length
⑧ Required pressure
⑨ Water quality
⑩ Voltage and frequency

The supplier can then select:

• Pump model
• Number of stages
• Motor power
• Cable size
• Control system

For more information about complete pump selection, read our submersible well pump selection guide.

Frequently Asked Questions

Can well-pump HP be selected from well depth?

No. Total well depth does not show the actual lifting requirement. Motor power must be based on the required flow and Total Dynamic Head, which includes the dynamic water level, elevation, outlet pressure and pipeline losses.

Does a higher-HP well pump deliver more water?

Not necessarily. Flow depends on the complete pump hydraulic design and the system resistance. Two pumps with the same horsepower may have very different flow and head curves.

Is an oversized well-pump motor safer?

Not automatically. A larger motor may increase purchase cost without improving the hydraulic match. An oversized pump can also create excessive pressure, high flow, well over-pumping and operation outside the recommended efficiency range.

Should pump efficiency or motor efficiency be used in the formula?

Pump efficiency is used to convert hydraulic power into the required pump shaft power. Motor efficiency is then used to estimate electrical input power. These two efficiencies should not be treated as the same value.

Is calculated horsepower the final motor size?

No. Calculated horsepower is a preliminary requirement. The final standard motor rating must be selected from the pump performance and power curves while considering motor load, operating range, voltage and manufacturer requirements.

Request a Submersible Well Pump Selection

To calculate the required pump and motor power, send SLAPK:

1. Required flow rate
2. Total Dynamic Head
3. Total well depth
4. Static and dynamic water levels
5. Well casing diameter
6. Pump installation depth
7. Pipe diameter, length and material
8. Required outlet pressure
9. Water quality and sand content
10. Voltage, frequency and phase
11. Starting method
12. Daily operating hours

Our engineering team can select a suitable submersible well pump, number of stages and motor rating for the actual project duty point.
The recommendation can include the performance curve, efficiency, shaft-power requirement, rated current, materials, dimensions and technical data needed to compare supplier quotations.

Request a Pump and Motor Selection