Running a well pump on solar

Mcsap Registered Users Posts: 5 ✭✭
Ok, I'm completely confused.  I'm brand new at this and electrical stuff makes my head hurt.  

I'm having a well drilled at deer camp.  It's a 1/2 HP 115 v pump.  I will not need to run it very much, just to fill a 33 gallon holding tank, probably once or twice a week if the smell gets bad and we want to use the shower.

I found for sale, brand new,  3 100W panels and a 100Ah 12 V AGM deep cycle battery.  I will need to purchase the inverter and controller.  

Is that battery enough to start and run the pump?  From what I have read, I will probably need two batteries this size?

Cheaper to just run the generator, but I like the idea of running silent.


  • BB.
    BB. Super Moderators, Administrators Posts: 33,478 admin
    Welcome to the forum Mcsap,

    The issue is that the loads pretty much drive the design and operation of the power system (solar, genset, etc.).

    So, can you tell us a bit more about your pump? Brand/model/type? Well depth, etc.

    Could you use a smaller pump--I.e., "slow pumping" to fill the cistern or pressure tank? And use a small 12 volt RV water pump to supply pressure to the camp (sink, shower, etc.).

    A 1/2 horse 120 VAC well pump does not sound like a lot of power--But for solar it is a pretty big load--Both starting (roughly 5x running watts for starting Watts/VA). And then how long to run (per day) to fill the tank.

    For starting surge, you need to size the battery bank and AC inverter to supply the surge. And the solar panels+battery bank to supply your daily loads (rough location of camp, nearest major city, hours of sun per day, is this a non-winter camp, or do you want to run in winter, etc.)...

    For example, here is a small in-well pump (in well pumps are not cheap)--But uses a (relatively) small amount of DC power to pump water (the one below, around 30-150 Watts--Depending on depth and operating voltage).

    With a DC pump, you can run with just a solar array--And save on battery bank and AC inverter.... Or, add battery bank+solar charge controller, and you can pump anytime day or night--Or even possibly during winter on occasion.

    If you have solar--Are there other small loads you would like to run? LED LIghting? Cell phone/radio charging? Will you have a backup genset/Need genset for other "larger jobs" (120 VAC saw(s) for construction and maintenance)?

    With solar, it is almost always less expensive to spend your money (and time) on conservation vs just building out a larger off grid power system to supply "less efficient" loads.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Mcsap
    Mcsap Registered Users Posts: 5 ✭✭
    The pump they said they were install is a Pentair Sta-Rite - 10gpm 115V 1/2HP pump.  They haven't drilled yet, so I don't have the depth.

    I do have a 12v pump that I use to pressure the holding tank.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,478 admin
    I am guessing similar to this one? (manual)

    The manual says minimum 14 AWG wiring and 35 amp standard fuse or 20 amp dual element fuse... Guessing we are looking at:

    This is a 2 wire pump motor which tend to have higher starting current requirements. @mike95490has a nice table from Franklin pumps which lists starting current for various types of pumps:

    Let's use the 55 Amps starting current. A good AC inverter will surge at least 2x the running wattage/VA.
    • 55 amps / 115 VAC = 6,325 Watt/VA starting surge
    • 6,325 VA surge / 2 = 3.162.5 Watts running
    So--You are looking at a ~3 kWatt AC inverter.

    A 3 kWatt inverter will pull at rated load and 12 VDC input:
    • 3,000 Watts * 1/0.85 AC inverter eff * 1/10.5 VDC battery cutoff = 336 Amps (and 2x that for starting surge)
    Pulling 336 Amps is a lot of current... And I would be suggesting, at least, a 24 VDC input AC inverter (good for ~2,400 to 3,600 Watts suggested maximum AC invert @ 24 VDC). And a 48 VDC battery bus would not be out of the question either (not overkill).

    For a flooded cell lead acid battery bank, normally suggest a minimum battery bank AH capacity for a 3 kWatt inverter of:
    • @48 VDC bus: 3 kWatt * 100 AH per 1 kWatt inverter rating = 300 AH @ 48 volts
    • @24 VDC bus: 3 kWatt * 200 AH per 1 kWatt inverter rating = 600 AH @ 24 volts
    • @12 VDC bus: 3 kWatt * 400 AH per 1 kWatt inverter rating = 1,200 AH @ 12 volts
    For a 24 VDC Bus, that would be (for example) 4x 6 volt @ 200 AH "Golf Cart" type batteries in series for 24 VDC and 3x parallel strings for 600 AH (24 volts @ 600 AH or a total of 12 "Golf Cart" batteries)...

    You are looking at using AGM batteries--Which do have somewhat higher surge current capabilities--Maybe cut bank size to 50-75% of FLA bank capacity... But that will be pretty hard on the smaller AGM bank... Will continue with FLA batteries for now (suggested for first time setup--FLA batteries are usually much less expensive and good for a "training bank"... (many folks "murder" their first set or two of batteries until they learn how to keep everything running).

    Two ways to size the solar array. First is rate of charge and second is based on location (amount of sunlight by season) and hours of load operation per day. In your case, it sounds like you are not using much water per day.

    Sizing solar array to properly charge the battery bank... 5% rate of charge for a "sunny weather" weekend/non-winter bank can work. 10%-13% is usually recommended by FLA battery manufactures:
    • 600 AH * 29 volts charging * 1/0.77 panel+charge controller derating * 0.05 rate of charge = 1,130 Watt array minimum
    • 600 AH * 29 volts charging * 1/0.77 panel+charge controller derating * 0.10 rate of charge = 2,260 Watt array nominal
    • 600 AH * 29 volts charging * 1/0.77 panel+charge controller derating * 0.13 rate of charge = 2,938 Watt rs array "typical" cost effective maximum
    And there is sizing the array based on location (hours of sun per day) and your loads... Say fixed array, facing south, in Michigan:

    Average Solar Insolation figures

    Measured in kWh/m2/day onto a solar panel set at a 47° angle from vertical:
    (For best year-round performance)

     Let's toss the bottom three months (not much sun) and use February as your "break even" month. And use 5% rate of charge array size:
    • 1,130 Watts * 0.52 off grid AC system eff * 3.56 hours of sun Feb average = 2,082 WH per day average Feb harvest (predicted)
    And pump runs at 12 amps (1.6 service factor):
    • 12 amps * 115 VAC = 1,380 Watts
    • 2,082 Watt*hours harvest / 1,380 Watt motor (this is "high" for various reasons--But lets use number for now) = 1.5 hours per day running
    • 1.5 hours per day * 60 minutes per hour * 10 GPM = 900 Gallons of water per average February day pumping
    This is a major issue with "solar unfriendly" loads. In this case, the motor draws a lot of power, your water needs are not very much--But in this case, have to "way oversize" the battery bank and solar array to supply the short term/high power loads (I guess you are needing closer to 30 gallons of water per day vs 900 GPD). (Of course, you only plan on using 50% to 65% per day of your solar harvest for "base loads" or loads you need to run daily--Allows for a few days of cloudy weather and such).

    This not a small system. And it is enough to run a nice off grid cabin (lights, fridge, "solar friendly" water pump, laptop computer, washing machine, etc.)...

    You probably would be better of with a "cheap" 4 kWatt genset running 5 minutes per day than solar with this planned well pump.

    If you used a "solar friendly" 24 VDC well pump and RV pressure pump--The system would be much smaller and cheaper (and make up for the "expensive DC well pump" cost).

    For example. Say 24 VDC well pump at 150 Watts @ 0.5 GPM. And 24 VDC RV pump at 5 amps (24v*5a=120watts) @ 3 GPM. And a 33 gallon per day water usage:
    • Peak power = 150 Watts + 120 Watts = 270 Watts peak
    • 270 Watts peak / 24 volts = 11.25 Amps
    • 11.25 Amps * 8 hour discharge rate (FLA battery) = 90 AH @ 24 VDC minimum bank based on peak running loads
    Now energy per day:
    • 33 gallon per day / 0.5 GPM = 66 Minutes per day well pump = 1.1 hours per day
    • 1.1 hours per day * 150 Watt well pump = 165 WH per day well pump
    • 33 gpd / 3 GPM RV pressure pump = 10 minutes per day = 0.2 hours per day
    • 0.2 hours per day RV pump * 120 Watts = 24 WH per day
    • 165 WH + 24 WH = 189 WH per day water pumping
    Assume 2 days of "no-sun/bad weather" pumping and 50% max planned battery discharge (for longer battery life):
    • 189 WH * 1/24 volt battery bank * 2 days storage * 1/0.50 max discharge = 31.5 AH @ 24 VDC bank sizing based on daily usage
    In this case, the 90 AH @ 24 VDC battery bank (based on supplying loads) would be a good fit (you could use down to 31.5 AH @ 24 VDC with AGM possibly--But that is a heavy load current for a small battery bank).

    Sizing battery bank based on bank size and rate of charge:
    • 90 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 169 Watt array minimum
    • 90 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 330 Watt array nominal
    • 90 AH * 29 volts charging * 1/0.77 panel+cotnroller derating * 0.13 rate of charge = 441 Watt array "typical" cost effective maximim
    And sizing the array based on location and daily loads:
    • 189 WH per day * 1/0.61 off grid DC system eff * 1/3.56 hours of sun per day Feb = 87 Watt array "February break even"
    So, a very reasonable array of 169 to 441 Watts (all solar numbers are "approximate values) with a 90 AH @ 24 VDC battery bank would be a nice fit. Get some 24 VDC LED compatible lighting and you are ready to go.

    This is why I/we highly suggest "conservation" first. Refrigerators and standard 120/240 VAC well pumps are large loads for an off grid solar power system. And trying to minimize those loads with the most efficient motors we can is a big help.

    Your thoughts?

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Mcsap
    Mcsap Registered Users Posts: 5 ✭✭
    Ok, I see what you are saying.  That is a lot of juice.  I appreciate the depth of your comments.  

    This is in the Upper Peninsula of Michigan, so sun hours can be rare in Nov.   I'd also want as few batteries as possible, since I have to pull them and take them home over the winter.  We have a propane frig and use propane lights, but electric lights would be nice since propane is so dim.  We use marine batteries for cell phones, wifi, and to run the 12 v pump. 

    I'll ask the well driller about a solar friendly pump, but I'm thinking that given how much I use the camp, I will probably just use the generator.  I will keep your notes, I might change my mind in a year or two.  Thanks very much.
  • wellbuilt
    wellbuilt Solar Expert Posts: 763 ✭✭✭✭
     I’ve been collecting water off my roof for 7 years now .
      I don’t really use much water , I use about 500 gallons a year +-  so I get 2 totes of water in the spring 275 each and the second tank still has water the following spring . 
     You could keep a large tank out side and just tee it in to the system . 
     My garage tank freezes some years and just defrosts in spring .
     Just leave the tank open with out a valve and it won’t crack the pipe .
     Turn off a valve in the house . 
      My pump is set at 700’ +- and it uses 700watts and will run on any power provided 
     no serge at all . I use a grundfus.  SQF 6-3  it make pumping water easy . 
      Have your driller watch for where your water is and set your pump as close to the surface
    as possible . 
      You could just leave your battery’s over the winter charge them up and turn it off or disconnect the battery cable. 
      There is little self discharge when the battery’s are cold . 
    Out back  flex power one  with out back 3648 inverter fm80 charge controler  flex net  mate 16 gc215 battery’s 4425 Watts solar .
  • Chris11
    Chris11 Registered Users Posts: 130 ✭✭
    I use, and have used since April of 2016, a 1/2 hp deep well pump on solar.  The well isn't that deep, 68 feet and the pump is about 7 feet above the bottom.  It's powered by a 2500 watt (or 3000 watt inverter, sorry my age interferes with my memory.)  Two 12 volt in-series marine so called deep cycle bats.  I have a 24 volt 185 watt single panel on the well house roof.  The controller is a Morningstar, again don't remember the model.  I know it was a very simple mppt.

    It pumps into a 35 psi on and 55 psi off pressure tank.  Then the water goes to a couple of spigots.  Occasionally I use a 5/8ths inch garden hose to go about 10 to 12 feet higher in head, length about 150 feet, to a couple of 55 gallon drums that feed my legal outhouse flush toilet.  I have to fill them a few times per summer depending on how many people use the outhouse. ( Duh.)

    I haven't had a single problem using this system.  I fire it up in the spring and shut it down for the winter.... the water part.  The solar just stays on year round.
    If I knew how to take a picture of the pump label off my cell phone and put it here I would.  The label reads...
    Hallmark Industries , Part number MA0343X-6.
    Flow max 20 gpm.   Volt 115AC
    Max head 125 feet   RPM 3450
    Discharge 1 1/4 NPT   HP 1/2
    Weight 23 lbs   Amp 7.3
    Capacitor 45 uF   Hz 60

    I bought this off Amazon, but from foggy memory I seem to remember it's been replaced with a new "improved"  Dunno how you can improve on the great service I've received off it.