How to size a solar water pump

Step 1: water per day, for the hottest day

Add up what the system has to deliver in a day. Size for hot weather, because that is when demand peaks and when a short tank hurts most. A beef cow can drink 16 to 21 gallons a day at 90F according to NDSU Extension; a horse at work in the heat can pass 20. For a house, the EPA puts average US indoor and outdoor use near 82 gallons per person per day, and a careful off-grid household uses a good deal less.

Write the total down in gallons per day. Everything else is built on it, and it is the number people most often guess low.

Step 2: sun hours in the month that matters

A peak sun hour is one kilowatt-hour of sunlight on a square metre, the equivalent of one hour of full noon sun. A direct-drive pump gets roughly that many hours of full-rate pumping a day. Use the month you most need water and have least sun. For year-round stock water that is December or January almost everywhere in the US; for an irrigation pump that only runs in summer, it is the summer average.

Our sun hours by state table gives the annual figure and every month, from NREL's National Solar Radiation Database.

Step 3: flow rate

Divide the daily volume by the pumping minutes. 400 gallons a day with 3.5 sun hours is 400 / (3.5 x 60) = 1.9 gallons per minute. That is the flow the pump has to hold at your head, not its headline maximum.

Step 4: total dynamic head

Total dynamic head (TDH) is the vertical lift from the water's pumping level to the highest outlet, plus the friction of the pipe at your flow, plus any pressure you want at the end. It is the single figure that decides which pump can do the job. Our total dynamic head guide works through it in detail, and the friction loss tables give the pipe term.

Step 5: pump watts, then panel watts

The physics is fixed: lifting water takes gpm x feet / 3,960 horsepower, or about 0.188 watts per gallon-per-minute per foot. Small DC pumps convert 25 to 50% of their electrical input into lifted water, so divide by that efficiency to get the pump's input. Then divide by about 0.75 for the array, to cover hot panels, dust, wiring and the weak morning and evening light a direct-drive pump cannot use.

A 120 W array sounds small, and it is: water is heavy but a cow does not drink very fast. Most real systems end up with two to four times this, because the pump that can make 91 ft of head at 1.8 gpm is rarely the most efficient pump at exactly that point, and because you want to fill the tank in the short days of December and still have margin. The calculation tells you the floor. The pump curve tells you the rest.

Where people go wrong

• Sizing to the pump's maximum flow. Maximum flow is at zero head. At your head, the same pump might deliver a third of it.
• Using static water level instead of pumping level. A well draws down when pumped. Ask the driller for the pumping level or the drawdown at a given rate, and use that.
• Forgetting the tank. A direct-drive pump makes no water at night or in a cloudy week. Three days of storage is a sensible floor for livestock.
• Using annual sun hours for a year-round load. The annual average in Minnesota is 4.4 hours; December is about 2.0. A system sized on 4.4 runs short every winter.