Choose a Solar Well Pump by Daily Volume, Head and Storage

A solar water pump for a well can supply a remote tank, livestock trough or household—but well depth and panel wattage alone cannot tell you which system will work. Selection starts with daily water demand, pumping water level, total dynamic head and the solar conditions during the critical month. Those are the core inputs in Grundfos’s solar-pump sizing guidance.

The first decision is whether to pump slowly into storage during daylight or deliver pressurized water whenever someone opens a tap. Those are different jobs, even when they use the same well.

Choose the delivery arrangement first

Daylight pumping into a storage tank separates water production from water use. The well pump fills a reservoir when solar power is available; stored water supplies demand at night or during poor sunlight. This arrangement can avoid batteries for the well pump. A tank-level switch stops pumping when the reservoir is full. Both the Pacific solar-pumping design guidelines and Grundfos’s SQFlex data booklet describe water storage in place of electrical storage.

Household pressure delivery needs another decision: can gravity provide enough pressure, or will a booster pump be required? An elevated tank does not automatically provide normal household pressure. For water, approximately 2.31 feet of elevation produces 1 psi; a tank water surface 20 feet above a tap provides only about 8.7 psi before pipe losses. The NYSERDA solar-pumping guide explains this conversion and the role of booster pumps.

A booster still needs power when water is used. Alternatively, a compatible solar well pump with appropriate pressure controls can feed a pressure system directly, but nighttime demand must be covered by usable stored water, batteries or a supported backup source. Do not mistake a pressure tank’s nominal size for a full-volume water reserve: part of the tank contains compressed air. Amtrol’s well-tank manual distinguishes tank size from drawdown—the water delivered between pump cycles.

For a tank-and-booster arrangement, use the water tank with a pump selection worksheet to define pressure, peak demand and reserve separately.

Measure water level, not just drilled depth

For a deep drilled well, a submersible pump is the usual starting point. Surface pumps drawing through a suction line suit shallow water sources; changing the power source to solar does not remove their suction limitations. The Pacific design guidelines distinguish these arrangements and call for a submersible pump that fits the borehole for deep-well installations.

Before requesting a selection, collect:

Site information	Why it matters
Well depth and casing inside diameter	Establish installation space and pump clearance
Static water level	Records the resting water level
Pumping water level at a known flow	Establishes lift while water is being withdrawn
Tested well yield and seasonal changes	Checks whether the source can sustain the proposed pumping
Pipe length, inside diameter and fittings	Allows friction-loss calculation
Discharge elevation and required pressure	Completes the hydraulic duty
Daily volume and peak use	Separates production needs from delivery needs
Array location and seasonal shading	Establishes the site’s solar exposure

The pumping water level includes drawdown: the lowering of the water surface during pumping. A 250-foot-deep well might have a pumping water level much nearer the surface. Do not substitute either drilled depth or pump-setting depth for that measurement. Colorado State University’s groundwater solar-pumping guidance bases lift on static water level plus drawdown and warns against selecting a pumping rate above sustained well yield.

Calculate total dynamic head

For a typical well-to-tank system, the practical calculation is:

Total dynamic head (TDH) = lift from pumping water level to discharge + pipe and fitting losses + required discharge-pressure head.

An atmospheric storage tank requires no added pressure head at its free discharge. A pressurized destination does. Horizontal pipe distance is not vertical lift, but it contributes friction loss. The NYSERDA guide’s hydraulic definitions explain both distinctions.

Consider these illustrative assumptions, not a ready-made pump specification:

• Pumping water level: 100 feet below the wellhead
• Tank inlet: 20 feet above the wellhead
• Calculated pipe and fitting losses at the proposed flow: 20 feet

TDH for filling that tank is 100 + 20 + 20 = 140 feet.

If instead the pump must supply an endpoint 20 feet above the wellhead at 50 psi, that pressure adds about 115.5 feet of head, making TDH approximately 255.5 feet, assuming the same flow and pipe losses. That change can materially alter pump and array selection.

Calculate friction using the actual pipe size and proposed flow. A blanket percentage allowance is not a substitute for that calculation on a long or restrictive pipeline.

Select daily production, then the pump and panels

A solar pump’s delivery changes with available power. Ask for gallons per day at your TDH, location and sizing month, not merely its maximum gallons per minute or maximum head.

Peak sun hours are an equivalent measure of solar energy, not the number of daylight hours or a guaranteed pump runtime. Mississippi State University Extension makes that distinction in its solar-pumping guide.

For a rough screening calculation, 600 gallons per day divided by four peak sun hours and 60 minutes gives 2.5 gpm. This expresses the required volume over an equivalent full-sun period; it does not establish that a particular pump will run at 2.5 gpm for four hours.

Final selection should use the manufacturer’s curves or solar-sizing software to account for changing power through the day. Grundfos’s sizing procedure provides daily and monthly production results and emphasizes choosing the month when the requirement is most critical. A year-round drinking-water system and a summer irrigation system may therefore need different sizing months.

Panel wattage alone is also insufficient. The proposed array must meet the pump/controller’s voltage and current requirements. More watts do not correct an incompatible voltage, as the Mississippi State guide’s array-sizing example explains. Have the supplier document the complete electrical match, including cable length and losses.

Decide reserve and backup explicitly

For livestock systems, Mississippi State Extension recommends storage for three days of average demand. At an assumed 600 gallons per day, that means 1,800 gallons of usable reserve. Treat this as a planning benchmark, not a guarantee against every cloudy spell or equipment failure.

Essential household supply needs a reserve and backup plan suited to local weather and acceptable outage duration. Specify how the tank will refill after a deficit while still meeting ongoing demand.

An existing AC well pump may be usable, but conversion is not simply connecting panels to its wires. For example, the Grundfos RSI data booklet permits existing systems only with compatible motor specifications and a motor suitable for variable-frequency-drive operation; its described system uses three-phase motors. That does not establish compatibility with an ordinary single-phase household pump. Use qualified installers for the electrical and well work, following the equipment manuals and local requirements.

What a complete quote should include

Request a written selection showing:

• Pump model, controller and performance at the calculated TDH
• Expected daily production for the critical month
• Array configuration and electrical compatibility
• Well-yield assumptions, dry-running protection and tank-full shutoff
• Usable storage, pressure delivery and backup operation
• Pipe, cable, mounting, disconnects, grounding and surge protection
• Freeze protection, installation labor, commissioning and service access

For drinking water, include a sanitary storage arrangement and a water-testing plan—not just pumping hardware. Solar power does not treat water. CDC well-water guidance emphasizes proper construction, maintenance and at least annual testing.

The most useful buying document is a site-specific production estimate backed by pump curves and a complete system scope. A listing that offers only “maximum lift,” “maximum flow” and panel watts leaves the central question unanswered: will this installation deliver enough usable water when you need it?