Solar Well Pumps: How They Work and How to Size One

A solar well pump is simpler than it sounds. Solar panels feed a small controller, the controller runs a pump down in the well, and the pump fills a storage tank while the sun is up. The tank carries you through the night and through cloudy days, so the pumping side usually has no battery at all.

People mean two different things by a solar well pump, and they have different answers:

• Putting in a pump built for solar. These are DC pumps that start gently and run slowly through the day. Most of this page is about choosing and sizing one.
• Running the AC pump you already have from solar or a generator. That is harder, for a reason that has nothing to do with the water, and it is covered at the end.

Three things to find out before you shop

Most of the decision is made by your well and your water use, not by the pump. Find these three first.

How far down the water stands, not how deep the well is

This is the mistake that wastes the most money, and it is easy to see why people make it. The depth of the well is the number an owner knows. It is on the drilling report, and it is what the driller quoted. How far down the water stands is a different number that most people have never looked up.

A 400 foot well with water standing at 80 feet is an 80 foot lift. Sizing it as a 400 foot lift makes the job look five times bigger, and the pump, the array and the cost all follow.

The figure you want is the static water level, and it is on the well log. If you do not have the log, a water level meter will tell you, and some counties keep well records you can request.

How wide the casing is

Most DC solar submersibles have bodies between three and four inches across. A two inch casing, which is what a packer well normally has, will not take them, and no amount of careful sizing gets around that.

That rules out most of the market before anything else matters. Measure the casing, or find it on the well log, before you read a single product page.

How much water you use in a day

Add up everyone and everything the pump has to supply.

• People. The USGS estimates each person uses about 80 to 100 gallons a day for indoor use at home.
• Livestock. Penn State Extension’s planning table gives, per animal per day, 12 gallons for a horse or a beef animal, 2 for a sheep or goat, 1.5 for a pig, and 9 gallons for every 100 chickens.
• A garden. It is easiest to think in inches of water. One inch over 100 square feet is about 62 gallons.

Animals drink more in hot weather, and more again when they are working or nursing. North Dakota State’s tables put a horse in moderate work at about 11 gallons a day at 68°F and nearly 22 at 95°F. Size for your hardest month: the most water you need against the least sun you will have.

How hard the pump has to work

Pump makers measure the job in head: the height, in feet, that the pump is effectively lifting water. Pressure and pipe friction count too, converted into extra feet. Add up five things and you have the total, which pump tables call total dynamic head.

• Depth to standing water. From the first step above.
• Drawdown. How far the water level drops once the pump is running. The well log gives it.
• Rise to the tank. How far above the wellhead the water has to go.
• Pipe friction. Water dragging along the inside of the pipe. It is small on short, wide pipe and large on long, thin pipe, and a friction loss table for your pipe gives it in feet.
• Pressure. Anything the pump has to push against at the end, such as a pressure tank. This one surprises people, and it gets its own section below.

A worked example

Made-up numbers, to show how it adds up. Water stands 60 feet down and drops 10 feet while the pump runs. The tank sits 15 feet above the wellhead, and the pipe adds 5 feet of friction. That is 90 feet of head into an open tank.

Lifting 100 gallons a day through 90 feet takes about 28 watt-hours of energy in the water itself. The pump draws more than that, because the controller, motor and pump all lose some on the way. One small pump’s maker publishes 40% efficiency at about that head, which makes it about 71 watt-hours. If your worst month’s sunlight adds up to 3 hours of full sun a day, known as 3 peak sun hours, that energy needs about 24 watts of panel.

But the pump also needs enough power to run at all. The same maker’s table calls for a 35 watt array at 92 feet, and every other pump checked called for more. So here the pump’s own needs, not the day’s energy, set the size of the array. Makers list the array a pump needs beside its flow for each head in their performance tables.

The pump sizing calculator does all of this with your own numbers, and once you have picked a pump it checks your array and your daily water against the maker’s figures.

Why solar systems pump into a storage tank

Every PSI of pressure the pump pushes against counts as 2.31 feet of head. That is a unit conversion: a PSI is 144 pounds on a square foot, and a cubic foot of water weighs about 62.3 pounds.

A common pressure switch turns the pump off at 50 PSI. That is about 116 feet of head, and no tape measure on the property will show it to you. In the worked example above, pumping into a pressure tank would take the pump from 90 feet of head to about 206.

How much that matters depends on the well. With water 20 feet down and another 15 feet of drawdown, rise and friction, the pressure is about three quarters of the pump’s work. With water 100 feet down it is about half, and at 200 feet about a third.

So solar systems usually pump into an open tank instead. The well pump only has to lift the water, so it can be much smaller, and weak sun just means the tank fills more slowly. The pressure does not disappear. A small pressure pump at the tank makes it later, but only for the water you use under pressure, and water for livestock, a garden or a gravity-fed line never needs it at all.

A tank also stores water far more cheaply than a battery stores electricity. A few days of water in a tank costs a fraction of a few days of power in a battery bank, so size the tank first and the rest of the system gets easier.

Tank or battery?

Some kits add a battery so the pump can run at night or whenever a tap opens. That works, but the battery costs more, wears out, and loses some energy on the way in and out. A tank usually does the same job for less. If you do want a battery in the system, size it with the system sizing calculator.

If your pump sits in a pit

A pit lets a jet pump sit closer to the water, which helps it pull the water up. It does not reduce the work, because the water still has to climb out of the pit on its way to the tank.

Running the pump you already have

An ordinary AC well pump can run from solar through an inverter, or from a generator. Starting it is the hard part. For a moment as it starts, the motor draws several times its running current, and the inverter or generator has to handle that surge, not just the running load.

How hard that starting figure is to find depends on the motor. Franklin Electric lists it for each of its submersible motors in its installation manual. The nameplate on my own pump, a 1 HP Goulds J10S jet pump, does not print it. The rules of thumb you will find online are guesses, not figures for your pump.

What you can do:

• Measure it. A clamp meter that captures inrush current gives you the real figure in a few minutes.
• Look for a NEMA code letter on the motor’s nameplate. NEMA’s table turns that letter, with the horsepower beside it, into a range for the starting draw. Mine carries code K, and sizing a generator for a well pump has the table and works it through.
• Take the surge away. A soft starter, or a pump built for solar that ramps up gently, stops the starting current from being the problem.

If it hums but will not start, check the capacitor first

A pump that hums, will not turn, or trips a breaker looks exactly like an undersized generator, and usually is not one. On a capacitor start motor the start capacitor wears out, and a failing one produces those same symptoms. Check it before buying a bigger generator to fix a part that costs a few dollars.

Where these numbers come from

The head arithmetic is unit conversion, and it is the same method pump makers use in their own sizing guides. The water use figures come from the USGS, Penn State Extension and North Dakota State University Extension. The 40% efficiency and the 35 watt array in the worked example are from SunPumps’ published table for its SDS-D-128 at 92 feet. The other array figures compared here came from SunPumps’ table for the SDS-T-128 and Pentair’s for the Shurflo 9300.

There is no starting current figure on this page, because the one that matters is your own motor’s, from its nameplate or its maker’s manual.

This page recommends no pumps. A recommendation needs each pump’s table read against the head it will actually work at, and casing width rules out most candidates first.

Once you have a head figure and a daily gallon figure, the electrical side is the same as any other load. The system sizing calculator turns a daily watt-hour figure into an array and a battery bank, and the appliance power budget shows what the rest of your system is already carrying. If you are weighing a generator against more storage, generators for off-grid systems covers why that machine charges a battery rather than running a house.