Solar Setup Help

Hi Guys,

Yes I am very new at this and want to make sure to install my newly coming system correctly.

1st: I am trying to completely run our water pump and a couple lights with a 240 watt solar system. We live in country so have well water. So I purchased (2) 120 watt solar panels from UL Solar, and a 20amp charge controller. Now what?......

1. How many batteries do you suggest, or how much amp's?

2. Do I run the batteries and keep 12 volt system, or do I turn it to 24 volt, or does that even matter?

3. Can a Inverter be installed so it runs consistantly? Our well pump house is in back yard, so do not want to have to turn on each time we need water. Just want the inverter to always be running and of course pump will kick in when we turn on faucets.

4. I suppose I should install a fuse box, which I am guessing is installed between the solar panels and the Charge Controller? If so what size fuse is recommended?

5. Any other tips greatly appreciated.

Thanks guys and sorry for the many noob questions.



  • KeithWHare
    KeithWHare Solar Expert Posts: 140 ✭✭✭
    Re: Solar Setup Help
    milezone wrote: »
    Hi Guys,

    Yes I am very new at this and want to make sure to install my newly coming system correctly.


    First, what size is your pump? Horsepower, watts, volts?

    Second, where are you located?

  • BB.
    BB. Super Moderators, Administrators Posts: 33,402 admin
    Re: Solar Setup Help

    For 240 watts of solar panel, the typical battery bank size is based on 5-13% rate of charge (20 Hour battery AH rating). Assuming 0.77 solar panel/charger efficiency:
    • 240 watts * 1/14.5 volts charging * 0.77 derating * 1/0.13 rate of charge = 98 AH @ 12 volts
    • 240 watts * 1/14.5 volts charging * 0.77 derating * 1/0.05 rate of charge =255 AH @ 12 volts
    Remember that Power = Volts * Current... So one 12 volt battery at 100 AH has the same energy as 2x 12 volt 50 AH batteries at 24 volts (paralleling batteries adds Amp*Hours; adding batteries in series adds voltage).

    Sizing the battery bank is best done based on your loads... Light loads (less than 1,200-2,000 watts maximum, short leads) can be done with 12 volts.

    If you are over 1,200-2,000 watts and/or need to send the current longer distances, then 24 or even 48 volt battery banks make more sense.

    Also, peak continuous current flow and maximum surge current matter for battery banks--Roughly some rules of thumb for flooded cell:
    • 5% to 13% rate of charge (C/20 to C/8 rate of charge--as above--recharge batteries quickly and mix electrolyte)
    • C/20 to C/10 "nominal" continuous current draw (battery is fairly efficient)
    • C/8 the maximum recommended continuous current flow (battery may overheat with more current, battery capacity is reduced at higher loads)
    • C/2.5 the maximum surge current--over this value and battery output voltage may collapse and not operate starting loads very reliably.
    In the end, we can design with a set of panels and this is the storage and load profile it will support.

    Or, knowing the loads, we work backwards to size of battery bank (AH + Bank Voltage ratings) and the amount of solar array needed to operate the loads (also based on your location/hours of sun by day/season).

    There are things that can be "adjusted"--For example, a pump that does not run very long but has high current requirements; look at an AGM based battery bank (sealed batteries, high output current ratings, work well into subzero weather, work better than flooded cell banks with smaller solar array wattages).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2
    System2 Posts: 6,290 admin
    Re: Solar Setup Help

    Hi Guys,

    Thanks for the kind words. We are located in SW Kentucky, so get plenty of sun here. Not quite the AZ sun, but lots of it I suppose.

    As for load I am not too sure, I believe it is around 10 amps, but have yet to get the meter out and test. Our well is about 100 feet deep.

    As for numbers, boy I am a bit green on this. In layman's terms am I able to pull this off with the 240 watt system I purchased? Thanks again, and sorry my math is just a bit rusty for all those numbers. :-)

    By the way I just purchased a fuse box, and got a 20amp fuse, that should be fine for this correct?

  • BB.
    BB. Super Moderators, Administrators Posts: 33,402 admin
    Re: Solar Setup Help


    Sorry for the blizzard of numbers--but it really helps us size your system to your loads. Solar PV off-grid power is very expensive--And over or undersizing your system can even cost you more money.

    Also, when you start talking about current--Remember that there is a (probably) 12 VDC and a 120 VAC side of the system...

    Roughly because Power=Voltage*Current--If your pump is 10 amps at 120 VAC, the 12 VDC size will be 100 amps or higher... So it is very important that we know exactly what you are talking about (AC/DC, 120 Volt / 12 volt / etc.).

    20 amp fuse * 12 volts = 240 watts of power
    20 amp fuse * 120 volts = 2,400 watts of power

    100-200 amps or so of current takes a lot of battery bank and copper to properly route that energy around your system.

    And lifestyle will really impact the design of your system and how much power you will expect from it...

    For a small cabin, laptop, stalite link, a few lamps and a DC ceiling fan--400-600 watts of panel keeps them happy.

    Other folks want ~100 kWH per month for a, more or less, feel of living in the modern age (lights, computer, internet, washer, well pump, radio, TV, etc..)... And you might need 1,500+ watts of panels and a 2-4x larger battery bank, inverters, backup genset, etc...

    There are some reasonably priced options that you can add to your system to help you estimate your power usage:
    • Kill-a-Watt Meter (120 VAC 15 amp volt/amps/watts/kWH/etc.)
    • Battery Monitor (measures current*time into and out of battery bank--Look at the Trimetric for lower cost unit).
    • Hydrometer (measure how much state of charge you use per day, see if battery bank is being properly recharged, etc. For flooded cell type batteries only, not Sealed/AGM types).
    • AC / DC Clamp Current Meter (like this one or similar).
    Power usage is a highly personal choice and situation--It is very difficult for us to guess at exactly what will fit your needs without more information.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset