A solar water pump is sized from the water, not from a panel promotion. The US Department of Agriculture’s 2010 technical note on small photovoltaic pumping, written for livestock and irrigation, uses twelve steps that start with how much water the operation needs and only later reach the modules. Daily volume divided by peak sun hours is the design flow. Total dynamic head is vertical lift plus pressure head plus friction in the pipe between the intake and the storage tank. The pump is chosen from a curve that can deliver that flow at that head. The array is then chosen to supply the pump’s power. Reverse the order, and the catalog wattage has nothing to push against.
The note is explicit that it is not a standalone design manual. Hydraulics still sit in the engineering handbook, and local conditions change the water requirement. A table of daily animal use in that note is an Oregon example, with a drier eastern Oregon column and a western one, adapted from an older Midwest handbook and adjusted upward. A milking cow at 20 to 25 gallons a day in that table is not a universal livestock figure and it is not a household domestic supply. Use it only as a picture of how the note wants the first step done: a volume, a season, a place.
Volume, source, and sun, before hardware
Step 1 is the water requirement, and the note says it varies through the year. Step 2 is the source. A well needs the static level, the drawdown at the pumping rate, and the water quality, taken from the well log, with a real well test if the log shows the well may not keep up. Drawdown is part of the lift. Ignoring it sizes the pump for a water level that disappears as soon as pumping starts. A pond, stream, or spring changes the layout again, because topography and the delivery point define the pipe.
Sun comes before the module count. The note tells the designer to account for slope, aspect, and shade from trees that will grow, and to put the array close to the pump so the wire stays short. Peak sun hours in the design-flow example are the insolation expressed as hours. Their arithmetic illustration is 1,310 gallons a day divided by 7.2 hours. That pair is an example in the note, not a target. Your flow is your gallons divided by your hours. Storage matters because the pump runs when the sun runs. A tank that holds the daily need lets animals drink after the array has stopped. Friction downstream of that tank, the note says, is usually a gravity problem and is not part of the pump’s head.
Head, curve, then panels
Total dynamic head is the sum of vertical lift, pressure head, and friction from intake to storage. The pump curve has to show the design flow at that head. The peak power read off that point is what the array must supply. The note warns that solar pump specifications change quickly and that the manufacturer’s sheet, not a generic watt-per-gallon rule, is the selection tool. When several modules are required, they are wired in series, parallel, or both to hit the pump’s voltage and current, and the powers add only as a check on the total, not as a substitute for the voltage match.
Mounting is a structural step of its own. The note’s standard post details apply only inside stated criteria. Outside them, it calls for a qualified engineer. That is the right place for wind and for a taller pole. It is not a place for a guessed foundation because the electrical sum looked small.
| USDA step | What you must know first | What it prevents |
|---|---|---|
| Water requirement | Daily volume, varying by season and local practice | An array sized to a brochure animal |
| Source and drawdown | Well log or a pump test; or the surface-water layout | Lift calculated at the static level only |
| Design flow | Daily volume divided by peak sun hours at the site | A pump that cannot finish the day before the sun goes |
| Total dynamic head | Lift, pressure, friction to the tank | A curve read at the wrong head |
| Pump, then array | Power from the curve, then modules matched in voltage and current | Panels chosen before the pump exists |
Inverter or no inverter
Many small solar pumps are DC and do not pass through an AC inverter. Adding an inverter because it feels like a complete solar system adds a conversion and a device that can fail. Use an inverter when the pump you selected is AC and no DC pump meets the head and flow. If you do, the inverter’s continuous rating has to cover the pump’s power from the curve, at the temperature where the inverter will sit. Grid-tied inverters are the wrong type off the grid: IEEE 1547-2018 requires them to stop when they island, within 2 seconds. A remote well has no grid to follow. Off-grid inverter limits are in sizing an off-grid inverter. A fishery that pumps for aeration rather than stock water has a different biological constraint, noted in fishery solar systems.
The array location rule in the USDA note is also an electrical rule. A long wire from a sunny hill to a pump in the draw is a voltage drop. The note’s advice to keep the array close is how that loss stays small. It is not a comment about appearances.
Frequently asked questions
How many watts of solar does a water pump need?
As many as the pump curve requires at your flow and your total dynamic head, after the daily volume is divided by the site’s peak sun hours. The USDA note refuses a shortcut that starts with the modules. The 1,310-gallon and 7.2-hour figures in the note are an example, not a standard well.
Can the Oregon livestock table be used anywhere?
No. The note presents it as regional guidance, adjusted for a drier climate, and tells the designer to apply local conditions. A different animal, a different season, or irrigation rather than stock water changes step 1.
Does the pump run at night?
Not from the sun alone. The note uses storage so delivery can continue after pumping stops. Night pumping without a grid needs a battery sized for that load, which is no longer the simple PV-direct design the twelve steps describe.
Sources and references
The twelve-step sequence, design flow, total dynamic head, regional water-use examples, shade, wire length, and pump curves: USDA NRCS Technical Note 28, Design of Small Photovoltaic Solar-Powered Water Pump Systems, October 2010. Why a grid-tied inverter will not run a remote well: NREL/Sandia IEEE 1547-2018 primer.
