Off-grid farm power: sizing PV plus storage for irrigation pumps

Key Takeaways

• Size the PV array against the worst sun month at the farm's location, not the annual average, then add 25-50% oversizing on top of that worst-month figure so the system still meets demand on moderately cloudy days.
• An inverter's continuous power rating must exceed the pump's running wattage, but the number that actually catches undersized systems is the motor starting surge, which can spike several times higher than steady-state draw.
• Many farms pump during daylight hours and store water in a tank rather than storing electricity in batteries, which is usually the cheaper design where the irrigation schedule can be shifted to match the solar window.
• Batteries become necessary specifically when power is needed outside the solar window, or when the same system also serves other farm loads (lighting, refrigeration, security) that don't stop at sundown.

The instinct with irrigation pump sizing is to look up the pump's rated wattage and size the inverter to match. That number is the easy part. What actually determines whether the system works is the combination of three other things: which month has the least sun, how much surge the pump draws on startup, and whether the farm can shift its irrigation schedule to daylight hours or genuinely needs power on demand around the clock.

Size the array for the worst month, then oversize again

Every off-grid solar design should be sized against the worst sun month at the specific location, typically December or January in the northern hemisphere, not the annual average irradiance figure (Anern, off-grid solar case study: myths vs reality on farms, retrieved 2026-09-10). A system sized to the annual average will underperform for several months a year, precisely when irrigation demand may still be running.

On top of the worst-month calculation, add 25-50% oversizing so the array still meets demand through moderately cloudy conditions, rather than only in clear-sky conditions (Anern, retrieved 2026-09-10). This isn't a safety margin in the abstract, it's a direct response to the fact that "worst month, clear sky" and "worst month, realistic weather" are two different numbers, and only the second one reflects what the farm will actually see.

The load assessment starts with daily energy, peak power, and duty cycle

A proper farm load assessment separates three distinct figures for every load, not just the pump: daily energy in kWh, peak power in kW including any motor starting surge, and the duty cycle, meaning how many hours a day the load actually runs and during which part of the day (Anern, solar drip irrigation sizing guide, retrieved 2026-09-10). Farms often carry a genuinely mixed load profile beyond irrigation: refrigeration, ventilation, lighting, electric fencing, and workshop tools all draw differently across the day, and a system sized only around the pump's numbers can still fail once these other loads are added.

Run the pump's full specification, running wattage, starting surge, and daily runtime hours, alongside every other farm load through the power bank runtime calculator to get a genuine total rather than treating irrigation as the whole system.

Why the inverter has to be sized for the surge, not the running load

The inverter's continuous power rating must exceed the pump's maximum power draw, and that maximum includes the surge current the motor pulls on startup, which can be a multiple of its steady-state running wattage (HUIJUE, solar water pump inverters and irrigation systems guide, retrieved 2026-09-10). An inverter sized only to the pump's running wattage will brown out or trip every time the pump starts, which in practice means the system "works" until the exact moment it's asked to do its actual job. Confirm the pump manufacturer's stated starting surge, not just its running wattage, before finalising inverter size.

Batteries: needed for some farm designs, not all of them

Many irrigation systems pump during daylight hours and store water in a tank for later use rather than storing electricity in a battery bank, which is frequently the lower-cost design where the farm's schedule can flex to match the solar window (Anern, retrieved 2026-09-10). Water storage is generally cheaper per kWh-equivalent than battery storage, so a farm that can genuinely schedule its irrigation around daylight hours often doesn't need battery capacity for the pump specifically.

Batteries become relevant once power is needed outside the solar window, for night-time pumping on a fixed schedule, or once the same system is asked to serve other loads, security lighting, refrigeration, that don't stop when the sun goes down. Design storage decisions around water tank capacity for livestock or human-use applications separately: livestock water storage is commonly sized for 1.5-3 days of demand, while human or irrigation use may call for 3-7 days, a different sizing question from the electrical battery bank entirely (Anern, retrieved 2026-09-10).

Frequently asked questions

Do I need batteries for a solar irrigation pump system?

Not necessarily. If the farm's irrigation schedule can run during daylight hours and water is stored in a tank rather than needed on demand around the clock, many systems pump directly from solar without battery storage. Batteries become necessary once power is needed outside the solar window or the system serves other loads that run after dark.

Why does my inverter need to be so much bigger than the pump's rated wattage?

Because the inverter has to handle the motor's starting surge, not just its steady running draw, and that surge can be several times the running wattage. An inverter sized only to running wattage will fail specifically at pump startup, the moment the system is actually asked to work.

Should I size my farm's solar array for an average sunny day or a worst-case month?

The worst sun month at your specific location, typically December or January in the northern hemisphere, then add 25-50% oversizing on top of that worst-month figure. Sizing to an annual average leaves the system underpowered for several months a year.

The bottom line

Irrigation pump sizing fails in predictable places: sizing the array to an average rather than the worst month, sizing the inverter to running wattage rather than starting surge, and defaulting to a battery bank when a water storage tank would do the same job more cheaply. Get the load assessment right first, separately for the pump and for every other farm load sharing the system, and the array, inverter, and storage decisions follow from that. Once that load list exists, WiserMonks' off-grid farm energy systems team can size the array, inverter and storage against it rather than against a generic per-acre benchmark.

Figures were verified on 10 September 2026 against published off-grid solar and irrigation sizing guides. Actual sizing depends on your farm's specific pump specifications, local solar resource, and irrigation schedule; run your own load audit before finalising a system design.