METHODS: Sizing a Solar Water-Pumping System

A pole-mounted PV array for water pumping.

A solar water-pumping system can be a cost-effective, dependable method for serving your remote watering needs, whether it’s for irrigation, animal grazing, or potable use. For more details, see "Solar-Powered Water Pumping”.

A farmer in western New York plans to use solar-pumping to water 120 beef cattle. The water source is a drilled well located at the base of the hill, 1,800 feet from utility power. A surface-mounted 1,500-gallon polyethylene storage tank was installed at the top of the hill. The tank’s inlet is 71 feet above the ground level at the wellhead. To connect to the well, 340 feet of 11/4-inch poly pipe is needed (this common size tends to work well with most flow rates). A float switch is installed in the tank and is wired to the pump controller.

The storage tank will gravity-feed four 300-gallon stock tanks that are 25 to 40 feet lower. There are mechanical float valves in each stock tank to regulate the water level. This is a seasonal operation from April through October—no freeze-proofing is needed.

Additional Specifications

Well depth: 180 ft.

Static water level (SWL): 35 ft.

Recovery rate: 14 gpm

Anticipated drawdown: None, if pumping rate is kept below recovery rate

Water requirements: 120 beef cattle at 15 gpd each = 1,800 gpd

Avg. daily sun-hours (annual* average): 4.1 (246 daily sun-minutes) *A seasonal sun-hour average could be used, but could result in overestimating the gpd produced.

Estimated pump flow: 10 gpm

Total dynamic head (TDH):

  • 35 ft. from SWL to ground level (pump is at 170 ft., but only the SWL counts)
  • 71 vertical ft. from well ground level to the storage tank inlet
  • Head loss from 170 ft. of 1 in. drop pipe in well: 2.93 per 100 ft. from table × 1.70 = 4.98 ft. (see table in “Solar-Powered Water Pumping,” this issue)
  •  Head loss from 340 ft. of 1.25 in. pipe to tank: (0.88 per 100 ft. from table × 3.4 = 2.99 ft.

Total TDH = 35 + 71 + 4.98 + 2.99 = 113.97 ft.

Pump sizing: A Grundfos sizing table or online calculator shows that the 11 SQF-2 pump will provide 10 gpm at 120 ft. and require 500 W. The calculator accounts for PV array losses, so there’s no need to upsize.

Output: 246 sun-minutes per day × 10 gpm = 2,460 gpd, which exceeds the water requirements.

Proposed Solar Water Pumping System

  • Grundfos 11 SQF-2 pump
  • Two 250 W modules in series (Vmp 62.2)
  • Pole-mounted PV array
  • CU200 pump controller with a Grundfos float switch in the storage tank

Comments (8)

nopgetome's picture

I have a Grundfo 3 SQF 2 solar pump and would like to know the number/wattage of solar panels needed. I am placing the pump 220' that footage includes bottom of well to the top of the water storage tank. The water storage tank and control unit,generator interface are 60' away from the well head. I see that I will have loss on electric and water flow, but need help in figuring that out. I need to get 2.5 gpm for a water well certification. Any information would be helpful, thanks

Fred Golden's picture

Kevin Shea,

You might want to buy a 2,000+ watt pure sine wave inverter, and keep using the 230 volt pump that is probably fairly in-efficient.

Or you can consider a DC power pump that can fill a tank in the daytime, and have it sized to meet your daily needs in say 5 hours of peak sunshine.

What I have heard others have done is leave the 230 volt pump in place, and then install another pump beside it, with a second pipe, say 2" diameter that the second pump will hang from. It is common to place two pumps within one well, especially a 8" diameter well.

Would it be practical to install a tank near the well head, then a pump and second tank at the 70 meters (about 280' elevation gain) location?

280 feet of elevation gain is about 140 PSI pressure. Pumps normally take a LOT of power to make that much pressure. Is there a requirement to move the water that high?

Would it be more practical to install a well at the top of the hill, and then have gravity on your side, so that the water is flowing downhill from the second new well, to the property? If you are pumping the water up hill by 200' elevation change, it would be much less expensive to have the water system at the bottom of the hill, and let the cows walk down the hill to get a drink.

My buddy in Mexico built some shallow lakes for the cows to drink from. What they did is get ahold of a large bull dozer. Then carve out a lake about 20' deep from the side of a hill. Then dig trenches, so that it had several creeks running into the new lake. It was more a 'depression' in the hill, than a lake, therefor not needing a dam permit, or anything. Is something like that possible? It would work well for both fire support and feeding livestock. It can also be filled with a few fish, if it stays wet all year long, or some frogs will enjoy the area and keep bugs at a minimum.

There are several sources for DC water pumps listed in this magazine. When it comes to sizing the system, with advertizing a 140 watt solar panel for $229, it is less expensive to error on the side of a larger wattage system than to run short of water if the water level drops to far.

Good luck with your project!

Kevin Shea's picture

Well, thank you. I will look for more answers. I am not an electrician or a mathematician, so I probably can not see the entire formula here. Let me explain my scenario so that perhaps someone can help with the calculation. I have

Motor Franklin Electric 3/4Hp - 230 Volts - 6o Hz. 4 ̈ - 3Ph
SubDrive 75 Franklin Electric 1
Breaker Box and Breaker 230V

to pump up water from an 8" deep well 180 feet to level, further to 800 meters distance/ 70 meters elevation higher in a 2" line with check valves. Currently, I use a 6500 watt generator to do the job. My question is, IF I could retrofit using the same pump system, How? Remember hacks, that I have a deep well pump with security looping. I will not be able to take that pump out for rewiring.

Fred Golden's picture

One must also take into account that the pump motor using the power is 170' below ground level, and there would be 200' of wire resistance involved too!

I wonder if they could relocate the storage tank, and watering tanks to a lower elevation, to reduce the amount of head pressure required by the pump? They added 35 PSI additional pressure because the storage tank is so high above the well head. Just a thought for the next person who might need a well for a home.

Allen Rainey_3's picture

I agree with Fred. In addition, the Grundfos pumps work more efficiently at higher DCV input. I would use 6-panel strings to get about 180V of working voltage - and would use 2 strings. The CU200 will use what ever solar is needed to pump the max gpm. Solar panels are inexpensive right now. It is better to have too much solar than not enough water.

If you use more solar than necessary - you do not have to worry about lowered water levels during drought conditions. All of our work is in developing countries. We do not want to go back & add more solar later.

Fred Golden's picture

My thought is that on cloudy days you might not reach 10 GPM. Especially from 10 am to 2 pm, as you expect to collect a total of 500 watts per hour for all 4 of those hours.

I would rather see spending another $100 on the solar panels, and perhaps install 3 each 200 watt panels, for a total of 600 watts, then you can be sure you will have a excess of power available at all times! This might become important if the water table falls a few feet, and now the water is say 55' below the surface, and you are pumping longer each day.

Home Power has had an advertiser selling 200 watt panels in the $250 each range for many years. The small increase in solar panel capacity is insignificant in this case, considering all the cost to install the system, trenches, and tanks.

I hope that it all works great this summer! But still I am worried about the lower water level, due to the extended drought in California these past few years.

techs's picture

We installed a similar system (Grundfos direct PV to cistern, float switch, gravity pressure) for a domestic potable water system to our off-grid home. Year-round occupancy, 4000ft ASL, summer 100degF, winter -20degF. Installer used Grundfos sizing guides for pump & panel choices to produce average 500gal/day to storage. I made a few different design choices which others might find of interest (or might find poorly considered ) -- not so much related to sizing as to overall system function.

Site allowed 132ft of head above house (plus house above wellhead, plus wellhead above SWL) with 1500ft of trench & pipe. I used it. Effective head is power stored while the sun shines and faucet pressure delivered when it doesn't. This allowed the water supply to be completely independent of the house PV budget (i.e. no pressure pump), and we didn't have to build or give up space for a pressure tank. The pump can be powered by AC if needed (dying pump, PV damage, fire emergency, etc).

Supply pipe (single run flows both directions) is 2" Schedule 80 PVC with brass gate valves at main control points. This eliminated the need for friction calculation of low flows on the uphill push, and maximized downhill flow for fire protection systems. It also provided some resilience against mechanical & frost damage knowing that a few segments would end up buried shallow due to rock.

Cistern is 3000gal PE, bedded 2ft. Half of capacity is for routine use; half is for fire protection and other emergency need (no agency protection for wildland or structure fire, house is sprinklered). Installed with air gap overflow to drain away from tank when float switch fails to operate properly. Exposed cistern froze. Cistern enclosed in unheated insulated building froze. Cistern enclosed in insulated building with a homemade solar hot air collector has done well to -20degF for a couple winters.

William Heidecker's picture

What is the estimated cost of this suggested system (for the pump and PV components only)?

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