Solar Well Pumps

Who a solar pump is for

A solar pump is a good fit if:

• the site has no grid power, or bringing power in would cost more than the pump
• the water goes into a stock tank, cistern or storage tank rather than straight to taps
• you want a second way to get water out of the well during long outages
• daytime use matches daytime sun, like irrigation or livestock

It's a poor fit as the only supply for a full-time household with no storage. Pressure and flow drop with the light, and cloudy weeks happen.

How a solar well pump works

Most residential solar systems are a submersible pump in the well, a controller near the wellhead and a set of panels. Franklin Electric's SubDrive SolarPAK, for example, packages a 4-inch Franklin submersible motor and pump, a solar controller and a flow switch. The controller uses maximum power point tracking (MPPT), which squeezes the most power out of the panels as the sun changes, and it soft-starts the motor to reduce water hammer.

Some controllers accept AC as well as solar. The SubDrive SolarPAK switches to an AC backup source automatically when needed, which is how a solar pump can also run from the grid or a generator on a dark day.

The pump moves water whenever there's enough light. So the storage is usually water, not batteries: a tank or cistern that the pump fills during the day, with gravity or a small booster pump supplying the house. Batteries can run a pump at night, but they add cost and upkeep.

Options compared

For most homes on the grid, a generator or battery backup on the existing pump is simpler than a second solar pump. Our page on backup power for a well pump covers sizing it, and hand pumps for wells covers the no-power option.

Sizing: flow, lift and panels

Solar pumps are sized the same way as any well pump, with one more step. You figure out how many gallons you need per day, how far the water has to be lifted, and then how much panel it takes to do that in the sun you get.

Franklin's packages show the range on offer. The SubDrive SolarPAK comes in 0.75, 1.5 and 3.0 hp, with flow ratings from 5 up to 90 gallons per minute. The Fhoton SolarPAK covers 0.50 to 1.5 hp at 3 to 90 gpm in its smaller models. Panels for the SubDrive SolarPAK need to be at least 320 watts each, and Franklin says a new array or a retrofit to an existing one is possible after a size and performance check.

How many panels you need depends on the pump, the lift and your location's sun, so use the manufacturer's selector or have a solar pump dealer run it. Be suspicious of anyone who quotes a panel count before asking your well depth.

What it can lift

Submersibles in general sit anywhere from 50 to 1,200 feet down a well. A solar pump's practical depth depends on the model and how much power the array delivers. Deeper water and higher pressure mean more horsepower and more panels, and the flow you get at a given depth drops as the sun fades. Ask for the pump curve at your actual pumping level, not the shallowest number in the brochure.

What to skip

• Cheap kits with no curve. If a kit doesn't publish flow at a given lift, you can't tell whether it'll reach your water.
• Solar straight to the pressure tank with no storage. Pressure will sag every time a cloud passes.
• Undersized arrays to save money. The pump will start late, stop early and deliver a fraction of what you planned.
• Utility pumps sold as well pumps. Small submersible utility pumps are for draining, not for lifting water from a deep well.

Solar stock-tank heaters and trough heaters come up in the same searches, but they're a separate product and don't pump water.

Install notes

• The well comes first. Get the depth, static level, pumping level and yield from the driller's log or a well contractor. A solar pump that pulls the well down runs dry just like any other pump.
• Protect it from dry running. Franklin's SolarPAK controllers include built-in diagnostics and protection, and the package includes a flow switch. Ask what your controller does when the well runs low.
• Mount the array where it gets full sun and keep the wire run to the controller reasonable. Long DC runs lose power.
• Plan the storage. A tank, a float switch to stop the pump when it's full, and frost protection on anything above ground.
• Hire the electrical work out. Arrays, disconnects and the pump circuit fall under the electrical code, and many areas need a permit.

Maintenance is light. Franklin lists its SolarPAK packages as needing no routine maintenance, though panels still need cleaning when dust or snow builds up.