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Newbie needs help powering a Dankoff Booster pump on small system

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  • Newbie needs help powering a Dankoff Booster pump on small system

    Hello! I am new to the forum and very new to solar. This question has become very long and I really appreciate if anyone has the time to sift through it...I'm very confused.

    I am working on setting up my off-grid home and property, and need some help figuring out how to power my water pumping setup. I've got it covered as far as pumping from the well into a 2500 gallon holding tank (with a Grunfos on 2 solar panels), but then need to get the water from the tank and into my house and garden with pressure. I am planning to buy the Dankoff Flowlight Booster Pump and a pressure tank, but am confused about how many panels and batteries and what size inverter I will need.
    I have a guess, and if anyone can tell me if I'm on the right track, I would appreciate it so much.
    Most people seem to power the Dankoff off their house's battery bank, but I don't yet have a solar panel/battery setup for my house and can't afford to get the big system yet. For now, I really urgently need the water, and would like to set up a small energy source for just this pump, that could ideally stay working as it's own system in the future.

    Ok, so the Dankoff comes in 12 Volt, 24 Volt, 48 Volt or 115 Volts AC. I have read that higher voltage can be more efficient, but don't want to spend too much on batteries, so was going to go with the 24 Volt. (If anyone thinks there's a better choice, I'm all ears).
    Here is the chart I'm looking at from Dankoff
    Screen Shot 2017-04-10 at 8.58.44 AM.png
    So, with the 24 Volt pump, the Amps Draw is 8, which would mean it takes 192 Watts. (All of the models are 192 Watts, then).
    I will get the biggest pressure tank I can to minimize strain on the pump, at least 120 gallons.

    My rough idea is to get two 12 Volt deep cycle marine batteries, to make the 24 volts. If they are rated at 100 Amp hours, does that mean they would give 2400 watt hours?
    2400 watt hours divided by .75 watt hours per gallon pumped = 3200 gallons. So, with the full 100 watt hours of the 24 volt battery bank, I could pump 3200 gallons, right?
    Also, 2400 watt hours could power a 192 watt pump for 12.5 hours, correct? So, at 4.3 gallons/minute that would also give me roughly 3200 gallons in those 12.5 hours.
    If my math and reasoning is correct, those batteries would seem sufficient to me (once the sun goes down I only really want water for baths and dishes -- no toilet in the house). However, the gallons seem pretty high, so I feel like I might be missing something...

    Then, I need solar panels and an inverter.
    If I had a 300 watt solar panel and a 300 watt inverter (Samlex SA 300 24 Volt), would that cover it? I live in Southern Oregon with great southern exposure.

    I used this: charge time (hours) = (Amp-hours removed x 1.15)/charge rate in Amps and found that if the batteries were down to 0% --> Charge time = 115 / (300watts/24V) = 115/12.5 = 9.2 hours
    So to completely charge the batteries back up from zero would take 9.2 hours, but I don't think I would ever use even half of the 3200 gallons potential in a day or two.

    I also want to make sure it's possible to plug a generator into the inverter to charge the batteries when needed. Is that pretty standard on inverters?


    Thank you so so much to anyone who has some ideas on this!!

    ~Lucy

  • #2
    If you run the pump directly from your batteries, you can avoid greatly increasing the plant for an inverter. An
    inverter wastes power when its running near full load, it wastes more when the load is light, and it really wastes
    power when the load is off, probably most of the time for that ap. An inverter means running an AC motor,
    which has a short but very big starting surge. An inverter would have to be sized to handle the surge, then run
    very lightly loaded (inefficiently) the rest of the time. Your batteries would have to be beefed up enough to
    handle an inverter far larger than normal load. Batteries easily handle a brief starting surge when directly
    feeding a DC motor, maximum efficiency.

    House use may run below 300 gallons a day, if you're not watering the garden. Bruce Roe

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