Solar Well Pump Sizing Calculator
Total dynamic head and daily energy for a solar water pump, sized on the depth to standing water rather than the depth of the well.
The USGS puts indoor home use at about 80 to 100 gallons per person per day. Add livestock and garden water on top.
Static water level from the well log. This is how far down the water sits, NOT how deep the well was drilled.
Used only to check the field above. Nothing is calculated from it.
How far the water level falls once the pump runs. On the well log, at a stated pumping rate.
Zero if the tank sits at ground level next to the well.
From a friction loss table for your pipe size, length and flow rate. Long thin pipe can add more than the lift does.
Uncheck if the pump fills an open storage tank at atmospheric pressure. This is usually the largest single decision on the page.
The upper figure on the pressure switch. A common residential switch is 30/50, so 50.
Controller, motor and pump end together. It changes along the pump's curve, so read it from the maker's chart or table at your head figure.
Not the annual average. A pump sized on an average delivers water in June and runs short in December.
Once you have picked a pump. Makers list the array a pump needs beside its flow for each head in their performance table.
From the same table. If it gives gallons per minute, multiply by 60.
Total dynamic head
236 ft
The figure a pump performance curve is read against. Choose a pump that delivers your daily gallons at this head.
- Of which tank pressure
- 116 ft (49%)
- Energy in the water
- 222 Wh per day
- Energy the pump draws
- 634 Wh per day
- Array for the day's energy
- 211 W
Into an open storage tank instead, the well pump needs 323 Wh per day against 634, which is 49% less, and it can be a much smaller pump. The pressure does not disappear: a small pressure pump at the tank makes it later, but only for the water you use under pressure. Water for livestock, a garden or a gravity line never needs it.
The array figure covers the day's energy, for a direct-drive system with no battery: the pump runs when the sun is up and the tank carries you overnight. A real pump also needs a minimum array to run as its maker's table shows, and that can be larger, especially for small daily volumes. Once you have picked a pump, enter the maker's figure above.
A battery-buffered pump needs round-trip losses added, and is sized through the system sizing calculator instead.
This sizes the energy, not the starting current. Whether a generator or inverter can start a conventional AC pump is decided by its locked rotor current. See solar well pumps for where to find it, and what to do about it.
Pressurizing to 50 PSI adds 116 feet of head, which is 49% of the total. Pumping into an open storage tank takes it off the well pump. A separate pressure pump then makes it, but only for the water used under pressure.
Depth to water, not depth of well
This is the error that wastes the most money on the subject, 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, it is what the driller quoted. The depth to standing water is a different number most people have never looked up.
A 400 foot well with water standing at 80 feet is an 80 foot lift. Treating it as a 400 foot lift multiplies the head by five, and the pump, the array and the budget all follow it up. 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 many counties keep well records you can request.
The optional well depth field on this page feeds nothing. It exists only to check the field above it.
Why the pressure tank can be most of the pump's work
Every PSI the pump has to deliver costs 2.31 feet of head. That is a unit conversion: a PSI is 144 pounds per square foot, water weighs about 62.3 pounds per cubic foot, and the ratio is 2.31.
A common residential pressure switch cuts out at 50 PSI, which is about 116 feet of head the pump works against, and it appears on no tape measure anywhere on the property. On a shallow well the proportions are startling: with water at 20 feet and another 15 feet of drawdown, rise and friction, the lift is about 35 feet and the pressure is 116, so the tank is roughly three quarters of the well pump's work. On deeper wells it matters less: about half with water 100 feet down, and about a third at 200.
Which is why the checkbox above matters more than any other input. Pumping into an open storage tank takes that head off the well pump, so the pump and its array can be much smaller. The pressure does not disappear: a small pressure pump at the tank makes it later, but only for the water used under pressure, and water for livestock, a garden or a gravity line never needs it. The calculator shows the difference for the well pump on your own numbers.
How each number is worked out
Total dynamic head is depth to standing water, plus drawdown, plus the rise to the tank, plus pipe friction, plus pressure head at 2.31 feet per PSI. It is the figure a pump performance curve is read against.
Energy in the water is weight times height. A US gallon weighs 8.34 pounds, so daily gallons times 8.34 times the head gives foot-pounds, and there are 2,655 foot-pounds in a watt-hour. Nothing about the pump enters this figure, because it is the work the water requires.
Energy the pump draws is that figure divided by wire-to-water efficiency, which covers the controller, the motor and the pump end together. It is much lower than people expect and it varies across a pump's curve, which is why this calculator asks for it rather than assuming one. Read it off the curve at the head you calculated.
Array for the day's energy is the daily energy divided by peak sun hours, for a system with no battery. That is the usual arrangement for solar pumping: the pump runs while the sun is up and the tank carries you overnight. A water tank stores the same days of autonomy far more cheaply than a battery bank does.
Array to plan on is the larger of that figure and the array the pump's maker lists at your head. A direct-drive pump does not run on whatever arrives, and on a small daily volume what it needs can be well above the energy figure. Makers list it beside the flow for each head in their performance tables.
Pumping time needed is your daily gallons divided by the pump's flow at your head. When that comes to more hours than your worst month's peak sun hours, the pump cannot deliver the water in that month, and the calculator says so.
What this does not tell you
Whether a generator or an inverter can start a conventional AC pump. That is decided by locked rotor current, which is a property of the motor rather than of the water. Franklin Electric lists it for its submersible motors, and on other pumps a NEMA code letter on the nameplate gives it as a range. Every rule of thumb you will find is a guess repeated until it reads like data.
Whether a pump will fit your well. Most DC solar submersibles have bodies of three to four inches, and a two inch casing will not take them. Measure the casing before reading a single product page.
Both are covered in solar well pumps: how they work and how to size one, along with what to do about the starting current.