Quick answer
A solar water pump needs battery storage or another power source after sunset if the pump itself must run then. But needing water in the evening does not always mean needing an evening pump. You may be able to pump into a tank during daylight and use that water later. The right choice depends on volume, delivery pressure, timing and the controller's actual operating modes.
A solar pump that performs well on a sunny afternoon may stop just when you arrive home to water the garden. Battery backup sounds like the obvious answer, but first decide what must happen after dark: filling a watering can, running drip irrigation, maintaining pressure or circulating water continuously.
Those are different jobs. Choosing the storage arrangement around the job can save unnecessary equipment and help you avoid a system that has a battery but still will not water when you want.
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Start with the evening water requirement
Write down the amount of water needed, when it must arrive and where it has to go. “Water the greenhouse” is difficult to size. “Deliver 120 litres through the installed irrigation network between 8pm and 9pm” gives you something to check.
Distinguish an occasional late watering session from a dependable overnight supply. A decorative fountain can usually stop without consequence. An essential water supply or critical circulation system needs a defined failure plan and suitable equipment.
For a garden or allotment, ask whether you could change the watering schedule or draw from stored water by hand. If you need automatic delivery while absent, confirm that the timer, valves and controller also operate without daylight.
Understand what kind of solar pump you are buying
| Arrangement | What it does | After sunset |
|---|---|---|
| Direct solar pumping | Uses available panel power through suitable controls. | No pumping once solar input is insufficient, unless another source is provided. |
| Solar pump with integrated battery | Stores energy within the manufacturer's system. | Depends on controller settings, stored charge and the supported schedule. |
| Pump supplied by a separate battery system | Uses a compatible battery, charging system and controls. | Can operate within energy, current and control limits. |
| Daytime pumping into water storage | Moves water while solar power is available. | Stored water may be accessible by gravity or manually; pressure may still require a pump. |
The phrase “battery backup” does not tell you how much usable energy is included or whether night operation is allowed. It may describe a small buffer against passing clouds rather than hours of scheduled pumping.
For example, Irrigatia's L-series instructions describe battery-powered pumping on a daylight schedule. Its Q-series instructions include an optional session after dusk. These are model-specific behaviours, not features you should assume every solar watering kit has.
Read the manual for the exact model and version before buying. Look for night mode, manual operation, timer restrictions, low-battery cut-off and what happens when charging has been poor.
Could you store water instead of electricity?
The FAO's solar-irrigation practice brief describes pumping into elevated water storage during sunshine as an alternative to electrical storage. That principle can apply on a smaller plot too, although space, support and pressure determine whether it is practical.
If you mainly fill watering cans, a tank with an accessible tap may meet your evening needs. If you want gravity-fed irrigation, confirm the equipment works at the available pressure and flow.
Gravity pressure depends on the vertical difference between the water surface and the outlet. Roughly one metre of height provides only 0.1bar before pipe and filter losses. A low water butt does not automatically provide the pressure required by sprinklers or standard irrigation components.
Check performance near the lowest intended water level, when gravity pressure is weakest. A larger tank at the same height increases stored volume, not pressure simply because it holds more litres.
Do not put a heavy tank on improvised high supports to obtain extra pressure. Water weighs about 1kg per litre, so a 500-litre store contains about half a tonne before the tank's weight. Use properly designed support.
Our comparison of linked water butts and one larger tank helps with the storage layout. A ground-level tank may still need a booster pump for pressurised evening delivery.
Check the pump at your actual lift and pressure
A pump's maximum flow and maximum head are not normally available together. You need the flow it delivers against your installation's total requirement: vertical lift, pipe and filter losses, plus any pressure required at the outlet.
Grundfos explains how pump curves relate flow and head. Ask the supplier for the expected operating point and electrical input there. Do not use hydraulic output or motor shaft power as though it were the electrical draw from your supply.
A long narrow hose, dirty filter or higher destination can change the delivered flow and pumping time. Our guide to lift, pressure and flow for water-butt irrigation covers the water side of that calculation.
Estimate the energy for one evening
Suppose your installed pump delivers a measured 240 litres per hour at the required pressure and draws 60W of electrical power while doing so. You need 120 litres after sunset:
- Runtime: 120 litres ÷ 240 litres/hour = 0.5 hours.
- Pump energy: 60W × 0.5 hours = 30Wh.
Now suppose the controller draws 1W continuously for 24 hours: another 24Wh. The total becomes 54Wh before any additional losses. In this example, standby use is almost as important as the pumping itself.
These are illustrative figures, not typical performance claims. Measure the whole system where practical. If measurement already includes the controller or conversion losses, do not add them again.
For an AC pump, include inverter losses and idle consumption over the time it stays on. For a DC pump, check voltage compatibility and any converter losses. Neither arrangement should be selected from wattage alone.
Turn the energy requirement into a battery estimate
Assume you want two days of the example's 54Wh daily requirement without recharging: 108Wh delivered to the loads. If you allow 90% delivery efficiency from the battery and use 80% of nominal battery energy, the arithmetic is:
108Wh ÷ (0.90 × 0.80) = 150Wh nominal capacity.
This is a starting estimate before extra allowance for ageing, temperature and uncertainty. The 80% and 90% figures are assumptions; use the actual battery and equipment specifications.
Nominal watt-hours are approximately nominal volts multiplied by amp-hours. A 12.8V, 20Ah battery therefore has 256Wh nominal energy. That calculation alone does not establish compatibility or guaranteed runtime.
The battery, protection system and any inverter must also handle running current and motor starting demand. A battery can have enough watt-hours yet trip because the pump asks for too much current at startup. Confirm starting behaviour with the pump supplier.
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Check whether solar can refill the battery
A larger battery delays running out; it does not correct a persistent charging shortfall. The panel must support the complete daily demand and restore the reserve after poor weather.
As an illustration, a 50W panel receiving the equivalent of two hours at rated output, with an assumed 70% net yield to the battery, would deliver 70Wh. Against a simplified 60Wh daily battery-energy requirement, only 10Wh remains for rebuilding reserve.
“Two hours” here means equivalent full-power sunshine, not two ordinary daylight hours. Actual yield depends on season, shading, orientation, temperature and charging behaviour. Use a site-specific estimate for the months you intend to pump.
Our panels-versus-battery guide explains why additional generation and additional storage solve different problems. Decide how quickly you need recovery after a cloudy spell.
Choose controls that protect the whole system
Look for low-water protection so the pump cannot run dry, a suitable tank-full stop where relevant, low-battery protection and an operating schedule that suits the task. Consider siphoning and unintended draining when the pump stops; follow the manufacturer's plumbing instructions.
Use the specified battery chemistry, charging profile and approved expansion arrangements. Do not attach a larger battery to a compact watering kit unless the manufacturer supports that change.
Temperature limits vary. Victron's Lithium Smart operating instructions, for example, specify charging-temperature limits for that battery range. Follow your own battery's requirements rather than assuming every lithium battery behaves identically in a cold shed.
Keep electrical equipment protected from water and condensation in locations permitted by its instructions. Larger or custom installations need appropriately specified cables, fuses, isolation and competent electrical installation. Test the system through several complete watering and recharging cycles before relying on it unattended.
When is battery backup worthwhile?
It is worth considering when after-dark pumping is genuinely needed, gravity cannot provide the required delivery, and you have enough solar generation to replace the energy used.
Water storage may be simpler when the pump's main job is transferring water and you can use the reserve without powered pressure. Daylight-only operation may be enough when watering can be rescheduled.
Compare the complete cost: battery replacement, controls and weather protection on one side; tank, base, pipework and available space on the other. Neither choice creates more source water. Check your shed roof's collection potential if rainwater supplies the pump.
Frequently asked questions
Will any solar pump work at night if I add a battery?
No. The pump voltage, controller, battery and charging arrangement must be compatible, and the controls must permit night operation.
How long will battery backup last?
It depends on usable energy, pumping power, runtime, standby loads and losses. Calculate the complete daily requirement and confirm it through testing.
Can a water tank replace the battery?
Sometimes. It works when water can be used manually or delivered at sufficient gravity pressure. A pressurised outlet may still need powered pumping.
Does battery backup guarantee watering through cloudy weather?
No. It provides a finite reserve. Specify the number of low-sun days you need and how the system will recover afterwards.
Choose around when the pump must run
Start with evening litres and delivery pressure, then calculate runtime, energy and recharge. Battery backup earns its place when it solves a real timing or pressure requirement. Where daytime pumping into usable storage meets the same need, compare that option before adding electrical storage.
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