Off-Grid Solar Water Pumps in the US: How a Battery-Free System Is Sized

Where there is no grid power at a well, pond or stock tank, a solar pump can lift water during daylight and fill a tank for use later. This guide summarizes a New Mexico State University extension circular on livestock water systems. The principles apply to other small off-grid uses, but the circular is written for livestock, so check your own needs.

The parts of a system

According to the NMSU circular, a solar pumping system has these parts:

Solar panels (PV) producing DC power. They should be tilted toward the sun and kept out of shade, because partial shade can cause large power losses.
A pump and motor, usually a DC submersible pump for wells.
A controller, which lets the pump start and run in weak light such as clouds, early morning or late afternoon.
A water-level sensor or float switch that stops the pump when the storage tank is full.
Wiring and protection: direct-burial UF wire, grounding, disconnects and lightning protection.
Pipe and fittings. The circular warns that "a common mistake is to oversize the piping," and says half-inch to two-inch pipe is typically enough.

Store water, not electricity

The circular's design uses a storage tank instead of batteries. It states that tank capacity should equal 3 to 10 days of water use: cloudy climates may need about 10 days, while a sunny area may need about 3 days for livestock. Tanks can last decades, while batteries need periodic replacement. In windy, hot or dry places, allow for evaporation from open storage.

What solar pumps can and cannot do

Solar pumps are low-volume. The circular gives an average of about 1 to 5 gallons per minute.
DC submersible well pumps are typically 12 V to more than 36 V, drawing about 3 to 5 A, up to about 1 horsepower.
Most solar pumps are positive-displacement types, which keep lifting as speed changes with light. Centrifugal types are also used.
The circular says DC pumps use one-third to one-half the energy of AC pumps, with average pump efficiencies of 25 to 35 percent.

Sizing steps (summary)

Total your daily water need, with a cushion for evaporation and growth.
Multiply by the days of storage you want.
Check the solar resource for your latitude, preferably winter values. The circular treats solar as viable above 3.0 kWh per square metre per day.
Calculate total dynamic head: static head (water level plus discharge elevation) plus friction losses. The circular says friction losses are typically 2 to 5 percent in a well-designed system.
Pick a pump for your flow and head, then size the array for its voltage and power. The circular's example derates modules by a factor of 0.80.

The circular's spreadsheet is limited to about 4 gpm and about 230 ft of head. Larger or deeper systems need a dealer's design.

Questions to ask a supplier

What flow will the pump deliver at my total dynamic head, in winter light?
How many days of storage do you recommend for my use?
What is the warranty on pump, controller and panels, and who services it locally?
What protection is included against lightning and dry running?

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General information only. Not electrical, plumbing, legal or code advice. Confirm details with a licensed professional and your local authority.