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.
A household is often 50 to 100 gallons per person per day. Livestock and irrigation are usually larger than the house.
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. Solar submersibles usually fall between 15 and 45%, and it changes across the pump curve. Read it off the curve at your head figure.
Not the annual average. A pump sized on an average delivers water in June and runs short in December.
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
- Direct-drive array
- 211 W
Delivering the same water into an open storage tank instead, and pressurizing separately from there, needs 323 Wh per day against 634. That is 49% less energy for the same water, and it is normally the largest saving available on a solar pump system.
The array figure is for a direct-drive system with no battery, which is the usual arrangement for solar pumping: the pump runs when the sun is up and the tank carries you overnight. 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start a conventional AC pump is decided by its locked rotor current, which manufacturers of residential pumps generally do not publish. See solar well pumps for why, 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 and pressurizing separately avoids it.
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 is usually most of the work
Every PSI the pump has to deliver costs 2.31 feet of head. That is not a rule of thumb, it 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 115 feet of head the pump works against and which appears on no tape measure anywhere on the property. Put that next to a shallow well and the proportions are startling: with water at 20 feet, a little drawdown and a short run, the lift is about 35 feet and the pressure is 115. The tank is roughly three quarters of the total work.
Which is why the checkbox above matters more than any other input.Pumping into an open storage tank and pressurizing separately from there, or gravity feeding if the site allows it, removes that head entirely. The calculator shows what it saves on your own numbers rather than asking you to take it on trust.
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.
Direct-drive array 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.
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, and manufacturers of residential pumps generally do not publish it. It appears on neither the nameplate nor the literature for common models. 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 insolar well pumps: what actually sets the size, along with why the missing specification is missing.