Running a pump straight from a solar panel

Yes, a panel can drive a pump with no battery. The pump runs when the sun shines and stops when it doesn't. That sounds limiting, and it is, but for filling a tank it can be enough.

How a direct setup works

Wikipedia lists the parts of a solar pump system. They are a panel array, a charge controller, a DC water pump, fuses or breakers, wiring and a water storage tank¹. DC means direct current, the kind of electricity a panel makes. Extension guides say DC pumps are common in these systems because the panels also produce DC².

The tank does the job a battery would do. Wikipedia says storing energy as water is better than storing it in batteries for solar pumps, since no step changes one form of energy to another¹. You store water, not electricity.

The limits of going without a battery

It only works in daylight. Panels give no or very little power at night and on cloudy, overcast days². With a battery, pumping isn't limited to daylight hours². Without one, it is.

Plan around the sun's best hours. One guide says to assume the pump will run only during peak sunlight hours³. A peak sunlight hour is defined as 1,000 watts of solar energy on a square metre for one hour². It isn't the same as the hours between sunrise and sunset. Count the water you need per day, then fit it into those peak hours.

You need a bigger tank. Because the pump stops at night, storage matters. The guide says the storage tank should hold at least three days' worth of demand, for evenings and cloudy days³. It was written for livestock, so scale it to your own use.

Very cold weather. The guide says solar pumping systems are not typically designed for extremely cold weather, which it puts at below about minus 20 °C (minus 4 °F). It adds that they can be insulated to handle colder temperatures³. Check the maker's temperature limits.

The pump and panel must match

This is where beginners get caught. A panel and pump can't be mixed at random. The solar array's output must match the pump's power needs². In one worked example, a pump needed at least 30 volts DC. The guide warns that a module outputting only 12 volts DC wouldn't run it². That pump was a large one, but the lesson applies to any size: read the voltage range on the label.

A controller sits between the two. Its first job is to match the output power the pump gets with the power the panels can supply¹. A guide strongly recommends buying the controller from the same manufacturer as the pump³.

If the pump is AC rather than DC, an inverter is needed to change the panel's DC power to AC¹. Check what voltage your pump expects before buying anything. The watts and watt-hours a pump draws shows the sums.

Mistakes to watch for

• Buying a panel first and a pump second, with no check that they match.
• Expecting water at night. A direct system has none unless the tank holds it.
• Sizing the tank to a sunny week. Size it for the dull days.

Safety and when to call an electrician

Panels, controllers and fuses all carry current. Isolate the system before you connect or disconnect any cable, follow the maker's manual for your equipment, and call a licensed electrician if you're unsure. This isn't routine DIY. One guide says that if you aren't comfortable with basic electrical concepts, it would be wise to contact an expert².

If a panel setup ever touches mains-voltage wiring or the grid, that's licensed electrical work in Australia, not DIY. NSW Fair Trading says no one may start electrical wiring work there without a licence, however small the job⁴. The Western Australian government says the same about changing fixed wiring: an unlicensed person must not do it⁵. Check the rules for your own state.

If you want pumping at night, the next step is a battery. Pumping from a tank to a tap shows one way to lay it out.