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Choosing the right water pump starts with one question: what does the pump need to do?
A pump for moving rainwater from an IBC tote to a garden hose has different needs than a pump supplying several house fixtures from a buried cistern. The right pump must provide enough water flow and pressure while staying within its lift, power, plumbing, and operating limits.
The main factors to consider are:
- Where the water comes from
- Where the water must go
- How much water you need
- How high the pump must lift it
- How much pressure you need
- Pipe and fitting sizes
- Available electrical power
- Water cleanliness
- Pump controls and dry-run protection
- Climate and freeze exposure
- Maintenance access
Getting these basics right is more important than simply choosing the largest pump you can find.
Start With What You Want the Pump to Do
First, define the job.
Common rainwater pump jobs include:
- Moving water from one tank to another
- Supplying a garden hose
- Running drip irrigation
- Operating sprinklers
- Feeding a pressure tank
- Supplying toilets or washing machines
- Moving water from an underground cistern
- Draining a tank for cleaning
- Transferring water between IBC totes
- Supplying a cabin or other small building
Each job has different flow and pressure needs.
For example, transferring water between tanks usually requires plenty of flow but not much pressure. A sprinkler system may need both good flow and much higher pressure.
Choose the pump around the job rather than around a horsepower number.
Determine How Much Flow You Need
Flow rate is the amount of water the pump can move over a certain period. It is commonly measured in gallons per minute, or GPM.
A garden drip system may need only a modest flow. Several sprinklers running at the same time may need much more.
If the pump supplies several outlets, think about which ones may run together.
For example, a system might need to support:
- One garden hose
- Two irrigation zones
- A toilet filling
- A washing machine filling
You do not necessarily add every possible fixture together. Instead, estimate the realistic peak demand.
Also remember that the flow number printed for a pump is often its maximum under favorable conditions. Actual flow usually drops as the pump has to push water higher or create more pressure.
That is why flow should always be considered together with head height.
Understand Head Height
Head height, often simply called "head," describes how much resistance the pump must work against.
Vertical height is an important part of it.
If water starts in a ground-level rain tank and must reach irrigation uphill, the pump has to overcome that elevation difference.
But height is not the only resistance.
Water also loses pressure as it travels through:
- Long pipes
- Small pipes
- Elbows
- Valves
- Filters
- Check valves
- Hose fittings
- Irrigation equipment
These losses are sometimes called friction loss.
The pump therefore needs enough capacity for the total dynamic head, which combines elevation and plumbing resistance while water is flowing.
You do not need to become a pump engineer for a simple system. But you should avoid choosing a pump only from the straight-line distance between the tank and the outlet.
A 100-foot horizontal pipe run is very different from lifting water 100 feet vertically.
Check the Pump Curve
One of the most useful tools when comparing pumps is the pump curve.
A pump curve shows how much water a pump can deliver at different head heights.
In simple terms:
- Low head usually means higher flow.
- Higher head usually means lower flow.
- Eventually the pump reaches a point where it can no longer provide useful flow.
Suppose a pump is advertised as capable of 15 GPM. That does not mean you will always receive 15 GPM.
At the head pressure created by your actual plumbing system, it may provide substantially less.
Look for the point on the manufacturer's pump curve that matches your required flow and estimated head.
Try not to design a system around the absolute maximum rating of the pump.
Decide How Much Pressure You Need
Flow and pressure are related, but they are not the same thing.
Flow is how much water is moving.
Pressure is the force pushing that water through the system.
A transfer pump may move a large amount of water without producing the pressure needed for household plumbing or sprinklers.
Pressure is commonly measured in pounds per square inch, or PSI.
Your required pressure depends on what the pump supplies.
For example:
- Open tank transfer may need little pressure.
- Drip irrigation usually operates at controlled lower pressure.
- Garden hoses may benefit from moderate pressure.
- Sprinklers may require more pressure.
- Household plumbing needs a suitable and controlled pressure range.
Check the operating requirements of the equipment connected after the pump.
Too little pressure can cause poor performance. Too much pressure can damage tubing, filters, valves, irrigation parts, and other equipment.
A pressure regulator may be needed where downstream equipment requires lower pressure.
Consider Where the Pump Will Sit
The location of the pump affects which pump style makes sense.
Above-Ground Pumps
An above-ground pump sits outside the tank and pulls water toward itself.
These pumps are easy to inspect and service, but suction has limits.
The pump should normally be placed as close to the water source as practical, especially when it must draw water upward.
Long suction pipes, air leaks, clogged strainers, and excessive lift can cause problems.
Submersible Pumps
A submersible pump operates underwater.
It pushes water rather than pulling it through a long suction line.
This can make submersible pumps useful for:
- Underground cisterns
- Deep tanks
- Installations where suction lift would be difficult
- Systems where pump noise is a concern
However, the pump must be suitable for continuous underwater use and accessible enough to remove for inspection or replacement.
Transfer Pumps
Transfer pumps are mainly designed to move water from one location to another.
They can work well for filling tanks, emptying tanks, or general water movement.
Do not assume that a high-flow transfer pump can also provide the steady pressure required by household plumbing.
Check the Suction Lift
Suction lift matters when an above-ground pump is higher than the water level.
The greater the vertical distance between the water surface and the pump, the harder it becomes for the pump to draw water.
Real systems also lose suction performance because of:
- Pipe friction
- Small suction lines
- Air leaks
- Dirty strainers
- Valves
- Warm water
- Pump condition
Keep the suction side short and simple when possible.
Follow the manufacturer's maximum suction-lift instructions rather than designing around a theoretical limit.
If the water source sits well below the pump, a submersible pump may be the more practical choice.
Match the Pipe and Connection Sizes
A good pump can perform poorly when connected to undersized plumbing.
Check the pump's:
- Inlet size
- Outlet size
- Thread type
- Hose or pipe compatibility
Also check the tank outlet.
Rain barrels, IBC totes, and cisterns may use very different fittings.
A bulkhead fitting is a fitting that passes through the wall of a tank and creates a sealed connection for pipe or hose.
You may need adapters between the tank, pump, filter, and irrigation plumbing.
Avoid reducing the suction line unnecessarily. A small suction line can restrict flow and increase the chance of poor pump performance.
Long pipe runs may also need larger pipe to reduce friction loss.
Consider the Water Quality
Rainwater can carry debris from the roof and collection system.
Possible contaminants include:
- Leaves
- Grit
- Insects
- Roof particles
- Sediment
- Organic matter
A pump designed for relatively clean water may be damaged or clogged if large debris reaches it.
Details about matching a pump to system requirements help evaluate likely priming faults behind unstable delivery pressure.
Good collection design can help.
Depending on the system, this may include:
- Gutter screens
- Leaf screens
- A first-flush device
- Tank inlet screening
- Settling
- A pump intake screen
A first flush system diverts some of the first roof runoff away from the tank. This helps reduce the amount of dirt and debris entering storage.
Filters placed after a pump can also create resistance. Include that pressure loss when sizing the pump.
Never assume pumping or filtering rainwater makes it safe to drink.
Drinking-water use requires a suitable collection system, appropriate treatment stages, proper maintenance, and current water testing. Local requirements may also apply.
Think About Sediment Near the Tank Bottom
The bottom of a rainwater tank often collects sediment over time.
Placing the pump intake directly in this material can increase clogging and wear.
Depending on the tank design, the intake may be positioned above the lowest sediment layer or use a floating intake that draws water from below the surface.
The exact arrangement should match the pump and tank design.
You should still inspect and clean the storage system as needed. Pump placement does not remove the need for tank maintenance.
Check the Available Power
Water pumps can use several power sources.
Common options include:
- Standard household AC electricity
- 12-volt DC systems
- 24-volt DC systems
- Solar-powered systems
- Generator-powered systems
Make sure your electrical supply can handle both the pump's running load and startup demand.
Motors can draw considerably more power when starting than while running.
For off-grid systems, also consider:
- Battery capacity
- Inverter capacity
- Solar production
- Pump run time
- Wiring distance
- Voltage drop
Electrical equipment around water requires extra care.
Use equipment rated for the installation environment and follow electrical safety requirements. Permanent wiring, outdoor circuits, and larger pumps may require a qualified electrician.
Look for Dry-Run Protection
A pump normally relies on water passing through it for proper operation and cooling.
If the tank runs empty, some pumps can continue operating without water. This is called dry running.
Dry running can damage certain pumps.
Dry-run protection can shut the pump off when water is unavailable.
This feature is especially useful with rainwater tanks because stored water levels can change quickly during dry periods.
Other possible protection devices include:
- Low-water float switches
- Level sensors
- Pump controllers
- Pressure switches
Do not assume every pump includes these features.
Decide How the Pump Should Turn On and Off
A basic transfer pump may use a manual switch.
Other systems may benefit from automatic controls.
For example, a pump serving a garden hose might start when the faucet opens and stop when it closes.
Household systems commonly use a pressure-based control system.
Possible components include:
- Pressure switches
- Pressure tanks
- Electronic pump controllers
- Flow switches
- Tank level controls
A pressure tank stores a small amount of water under pressure. It can reduce how often some pumps start and stop.
Frequent starting and stopping is called short cycling. It can increase wear on pumps and controls.
The correct control system depends on the pump type and how the water will be used.
Consider Pump Run Time
Some pumps can run for long periods. Others are intended for shorter operating cycles.
Check the manufacturer's duty rating.
A pump that works well for occasionally filling a watering can may not be suitable for running irrigation for several hours.
Consider how long the pump will normally run and how often it will start.
Check Noise and Installation Location
Above-ground pumps can create noticeable noise and vibration.
If the pump will sit near a house, patio, or sleeping area, noise may matter.
A stable mounting surface can help reduce movement.
Do not completely seal a pump inside a small box unless the enclosure still provides the ventilation required by the manufacturer.
Electric motors can generate heat.
Also keep the pump accessible. Pumps, filters, check valves, and fittings eventually need inspection or service.
Plan for Freezing Weather
Water left inside pumps, filters, pipes, and valves can freeze.
Freezing water expands and can damage equipment.
If your system operates in a freezing climate, consider how exposed parts will be protected or taken out of service during cold weather.
Options depend on the system and may include:
- Locating equipment in protected spaces
- Designing plumbing so exposed sections can drain
- Disconnecting seasonal pumps
- Winterizing irrigation lines
Do not assume insulation alone guarantees freeze protection.
Follow the manufacturer's winter storage and draining instructions for the pump.
Do Not Oversize the Pump Without a Reason
A larger pump is not automatically better.
An oversized pump can create:
- Excessive pressure
- Higher power use
- More noise
- Frequent cycling
- Problems with filters or irrigation equipment
- A need for additional pressure regulation
A pump should have enough capacity to meet the required flow and head with some reasonable operating margin.
It does not need to be dramatically larger than the system requires.
A Simple Pump-Sizing Checklist
Before choosing a pump, write down the basic details of your system.
| Question | What to Check |
|---|---|
| What is the water source? | Barrel, IBC tote, cistern, pond, or other storage |
| What will use the water? | Hose, drip line, sprinkler, transfer line, or plumbing |
| How much water is needed? | Required flow in GPM |
| How high must water move? | Vertical elevation difference |
| How far must it travel? | Pipe or hose length |
| What pressure is needed? | Requirements of downstream equipment |
| Where will the pump sit? | Above water level or submerged |
| What pipe sizes are used? | Tank outlet, suction pipe, and discharge pipe |
| How clean is the water? | Sediment and debris that could reach the pump |
| What power is available? | AC, DC, solar, battery, or generator |
| Can the tank run empty? | Consider dry-run protection |
| Will temperatures freeze? | Plan winter protection or draining |
Once you have those numbers, compare them with the manufacturer's pump curve and installation requirements.
When Pump Sizing Becomes More Complicated
Simple tank-transfer and garden systems are often manageable as DIY projects.
Professional help becomes more useful when the pump will serve:
- Household plumbing
- Multiple irrigation zones
- Large elevation changes
- Long underground pipe runs
- Complex pressure systems
- Drinking-water systems
- Permanent electrical circuits
- Large underground cisterns
- Automatic backup water supplies
A qualified pump or plumbing professional can calculate pressure losses and select equipment that works together correctly.
This is especially important when an incorrectly sized pump could damage plumbing, run dry, overload electrical equipment, or leave the system without enough water pressure.
Frequently Asked Questions
Is a bigger water pump always better?
No. A larger pump may create more pressure and flow than the system needs. That can increase power use, noise, cycling, and stress on filters, pipes, and irrigation equipment. Choose a pump based on the required flow and total head.
How do I know what size pump I need for a rainwater tank?
Determine the required flow, vertical lift, pipe length, pressure requirement, and plumbing restrictions. Then compare those needs with the manufacturer's pump curve. The pump should provide the required flow at your system's actual head, not just at its maximum advertised flow.
Can I connect a pump directly to a rain barrel?
Sometimes. The barrel must have a suitable outlet and enough water to supply the pump. Check fitting sizes, pump suction requirements, and whether the pump has dry-run protection. Small rain barrels can empty quickly when connected to a high-flow pump.
Is a submersible pump better than an external pump?
Neither type is always better. Submersible pumps are useful when the pump needs to operate inside a tank or underground cistern. External pumps can be easier to access and service. The best choice depends on tank location, suction lift, flow, pressure, maintenance access, and power.
What is the difference between pump flow and pump pressure?
Flow is the amount of water moving through the system, usually measured in gallons per minute. Pressure is the force pushing the water. A pump can have high flow without producing enough pressure for sprinklers or household plumbing.
What happens if a water pump runs without water?
Some pumps can be damaged by dry running because water helps cool or lubricate their internal parts. A low-water switch, float, or dry-run controller can help protect a pump when a rainwater tank becomes empty.
Do I need a filter before my rainwater pump?
It depends on the pump and water source. Large debris should generally be kept out of the pump. Roof screens, tank screens, intake strainers, settling, and other prefiltration methods can help. Very fine filters can restrict pump suction, so filter placement should follow the pump and filter manufacturers' instructions.
