Well Pump Sizing Calculator

Sizing solar for a well pump

Running a well pump on solar is a classic off-grid and rural application — and a demanding one, because pumps are motor loads with two distinct power figures you must respect. There's the steady running wattage the pump draws while operating, and the much larger starting surge it demands for a fraction of a second when the motor kicks in, often two to three times the running figure. Your inverter (and battery or generator) has to supply that surge or the pump simply won't start, which is the most common reason a solar-pump setup that "should" work doesn't.

For rural properties, farms and off-grid homes, water and power often arrive as the same problem: no mains electricity usually means no mains water either, so the well pump becomes one of the most important loads a solar system has to serve. It’s also one of the trickiest, because a pump is a motor, and motors don’t behave like the steady electronic loads that dominate a typical home. They demand a brief but large jolt of power to start spinning, then settle to a lower running draw — and a system sized only for the running figure will fail at the worst moment, leaving you without water. Understanding that distinction, and choosing between a battery-backed system and direct solar pumping into a storage tank, is what separates a reliable solar water supply from a frustrating one. The calculator below helps you size both the energy and the surge sides of the problem.

Two ways to power a pump with solar

• Battery-based AC system. A standard pump runs from an inverter fed by a solar-charged battery. This gives water on demand day or night, but the inverter must handle the pump's surge and the battery must store enough energy for your daily water use plus reserve.
• Direct solar (array-direct) pumping. Common for agricultural and livestock water, the pump runs directly from panels (often via a suitable controller) while the sun shines, pumping into a storage tank. Here the "battery" is the tank of water — store water rather than electricity. This is simpler and cheaper for daytime irrigation or tank-filling where you don't need water at night.

Working out your daily energy

Estimate how many hours a day the pump runs (from your water use and the pump's flow rate), multiply by its running wattage for daily watt-hours, then size the array and any battery from that — as in any load calculation. Water-heavy uses like irrigation can mean long run times and a substantial array. For array-direct systems, you instead size the array to deliver enough pumping during sun hours to fill your tank for the day's needs.

Surge is the sizing trap

The running watts decide your daily energy (and thus array and battery size), but the starting surge decides your inverter size. A pump that runs at 1,000 W may surge to 2,500–3,000 W for an instant at startup; an inverter rated only for the running figure will trip. Always size the inverter (or generator) to the surge, and check the pump's specification for its locked-rotor or starting current. Deep wells and higher pressures mean bigger pumps and bigger surges, so this matters more the deeper you pump.

Storage tanks: the simplest battery

One insight transforms solar water pumping: a storage tank is often a better and cheaper "battery" than an electrical one. Instead of storing electricity to pump water on demand, you pump water while the sun shines and store it elevated or in a cistern, then draw on it any time by gravity or a small pressure pump. This sidesteps the cost, weight and surge-handling of a battery bank entirely, and it's how most agricultural and livestock systems work. If your use allows daytime pumping into storage, sizing the system becomes far simpler — match the array to the daily water volume, and let the tank handle timing.

Frequently asked questions

It depends on the pump's wattage and how many hours a day it runs. A small shallow-well pump running briefly needs only a few hundred watt-hours a day; a deep-well pump irrigating for hours can need a multi-kW array. Enter your pump's figures and run time above for an estimate.

Only if you need water when the sun isn't shining. For daytime irrigation or tank-filling, array-direct pumping into a storage tank avoids batteries entirely — you store water instead of electricity. For on-demand household water day and night, you'll want a battery-backed system.

Almost always because the inverter can't supply the pump's starting surge, which is much higher than its running draw. Size the inverter to the surge (often 2–3× running watts), not just the running figure, and check the pump's starting-current specification.

Excellent — daytime pumping aligns with sunshine, and storing water in a tank removes the need for batteries. Many farm and livestock systems use array-direct pumps sized to fill a tank during sun hours, which is simple, robust and low-maintenance.

For daytime, tank-filling duty, a purpose-built DC solar pump (array-direct) is often simpler and more efficient, with no inverter or battery to size for surge. For on-demand household water day and night, an AC pump on a battery-backed inverter is more flexible. The right choice depends on whether you can store water in a tank or need pressure on demand.