Need to pump 8 gal/min from a tank

phredophredo Registered Users Posts: 11
I have ac electricity at present but I want to design an energy efficient pumping system that could work from solar in the future for my new water well. My present plan is to use a Simple Pump ( with an ac to dc converter to pump from the well. But since it only delivers about 2 gal/minute and I would like to have about 8-10 gal/min for longer than the largest drawdown (~ 22 gal) pressure tank will give, I'm planning to pump into a holding tank and use another pump to pressurize to a pressure tank in order to get a continuous 8-10 gal/min at, say, 50 psi. But all the "solar type" pumps I find, basically Shurflo and Dankof, will give about 5 gal/min at most, and some of them are pretty expensive, too.

I could use either 12 v dc, from the same Iota charger as the simple pump will use, or ac, but if I use ac I don't want tooo high an amperage need. I think what I need is just a sort of "suction" pump, like the Shurflo "Shurflo Extreme smart sensor 5.7 RV pump 12 Volt", but that will give more gal/min without costing too much, but I haven't had much success looking online, maybe because I'm not exactly sure what to search for. Does anyone have ideas, including any comments or criticisms of my general scheme?

Thanks, Fred in northern California


  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Need to pump 8 gal/min from a tank

    i think this may depend on your reasons and intentions with this. normally, it would be easiest and most efficient to grid tie some pv strings. if you are thinking of offgrid then i would ask, why are you experiencing power outages or is there some other reasoning here? for power outages it still would be better to have a gt system, but this could be a battery backed gt system. this isn't as efficient and does add costs while allowing for periods of no power from the utility. to go totally offgrid for whatever reason would mean even greater costs in batteries and pvs to insure enough power is available to you.
  • BB.BB. Super Moderators, Administrators Posts: 33,319 admin
    Re: Need to pump 8 gal/min from a tank

    10 gallons a minute at 50 PSI... Using an engineering page like this for some back of the envelop calculations:
    • PSI= Ft of head/2.31
    • Pump HP= GPM x P (Ft.H.D) / [3960 x (0.65 to 0.75)] (Pump Eff)
    So, plugging in the numbers and assuming 65% efficiency (conservative estimate):
    • Ft of Head = PSI/2.31 = 50 PSI / 2.31 = 21.65 ft
    • Pump HP = 50 gpm * 21.65 / [3,960 * 0.65] = 0.42 HP
    1/2 hp does not sound like too large of load for solar/off-grid... But, now we need to know how much water you need to pump... A few hundred gallons per day or:
    • 50 gpm * 60 min per hour * 24 hours per day = 72,000 gallons
    Obviously, the size of the array and battery bank is drastically different between the two requirements.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • phredophredo Registered Users Posts: 11
    Re: Need to pump 8 gal/min from a tank

    "i would ask, why are you experiencing power outages or is there some other reasoning here?" At present, our small town in northern California tends to experience power outages from time to time when the weather is windy and rainy. But I also want to look toward the future when electricity costs may skyrocket or the grid may brownout for long periods.

    "Obviously, the size of the array and battery bank is drastically different between the two requirements." I'm not actually looking for a panel-battery solution at this time, just for a pump. I would like it to be compatible with panels and batteries in the future, that is, be as low in energy demand as possible.

    Perhaps I need a 1/2 hp pump to get 10 gal/min. I was guessing it would be a bit smaller. In that case, I would like to get something like the Grundfos SQ pumps with the "soft start" feature, meaning that they do not have high amperage surge upon starting up, unlike most AC pumps. But those pumps are submersible for placing in the well, and I need some kind of pump that will go from tank to pressure tank.

    You might ask, "Why then not just use the Grundfos SQ in your well to start with?" Three reasons: 1) a pump installer whose opinion I respect says he has had trouble with them because they are somewhat delicate and get damaged easily from any silt or sand in the well, and 2) I really want to have the hand-pumping feature of the Simple Pump for when the grid goes down, and, also, 3) I think I could install the Simple Pump easily myself and I would want to get an installer for a submersible pump.

    I don't really need 10 gal/min for very long at a time, as our household needs are modest. I am mostly wanting it for when I am gardening and want to use a hose for hard to reach places where I don't have a soaker hose. So one option I am seriously considering is to just put three (or more) 22 gal drawdown pressure tanks in a row, and that would give me about 8 minutes of watering from a 1/2" or 3/4" garden hose with some pressure (approx. 8 gal/min ?). Then I could forget about the additional pump and the big water tank and just let the Simple Pump pressurize the tanks at 2 gal/min. Of course, I'd be giving up "unlimited" 8-10 gal/min, but maybe I should just settle for that. Cheaper and easier, if not very "elegant."

  • westbranchwestbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: Need to pump 8 gal/min from a tank

    Phredo, I think you may be well overestimating you flow rate.. maybe by a factor of 2

    our house functions very well on less than 5 GPM, including underground sprinklers. Most 1/2 inch water lines will deliver about 2 - 4 gpm to a garden hose depending on the pressure setting for the pump. You don't need to rum all the hoses at the same time...

    A lower delivery rate will make things a lot cheaper.

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  • phredophredo Registered Users Posts: 11
    Re: Need to pump 8 gal/min from a tank

    "our house functions very well on less than 5 GPM"

    Thanks. I'll consider that.
  • Ralph DayRalph Day Solar Expert Posts: 1,018 ✭✭✭✭
    Re: Need to pump 8 gal/min from a tank

    My house systems are a Grundfos submersible at 10gpm in the well, and a Dankoff booster pump at 2.5gpm from the holding tank. The well supplies the house over the winter and the rainwater tank and the Dankoff over the other 8 months of the year. 2 systems because of low well supply over the summer months, and rain water is soooo soft!

    2.5gpm is enough for any application in our house. If you're showering you don't want laundry filling up or a toilet flushed anyway (even with 10gpm), so why go for the high delivery if it will seldom if ever be required? Probably a cheaper solution at any rate. Good luck,.

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