RV Solar System Design

coulter6coulter6 Registered Users Posts: 3
I'm considering adding a solar system to my motorhome and would like your feedback on the basic design.

My current system is a standard RV type 12v electrical system with 4 6v house batteries for a total of 440amps, a 1000w Xantrex 458 inverter/charger and Onan 5.5k generator with auto switching. We have a 27" and 20" TV that runs off the inverter. 2 AC units and a combination oven/convection/microwave that's wired to operate when plugged in or off the generator (not hooked up to inverter). I use a CPAP machine every night that draws 3 amps/hr. The inverter is wired to select appliances and outlets and is not a whole house inverter. The inverter has a remote on/off switch and temperature sensor. We do camp without hookups often and don't like to run the generator everyday to recharge the batteries. If we do run the microwave or AC, we'll use the generator. We use about 95 amps a day.

Here's my draft system:

4 Kyocera KC135 135 watt panels
2 panels wired in series (and those 2 groups wired in parallel) for a 24v system
1 Blue Sky Solar Boost 3024iL
IPN-Pro remote monitor
flat mounted on roof

From what I've read, a 24v system is more efficient and requires smaller gauge wire than a 12v system. Is there a significant difference or advantage in wiring a 24v vs a 12v system?

You're experience and comments will be appreciated.
Bob C.


  • mike95490mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: RV Solar System Design

    Well, if you convert to a 24V system you will have to toss the inverter and chargers and lights that you have that run/supply 12V That would be messy and expensive.
    You would only gain what you loose in the 12V wires from the battery to inverter.
    Upgrading your inverter - battery wires would be a better investment. To gain more storage, you would need more batteries, re-arranging the series/parallel without increasing the amount of lead & acid, just changes the voltage.

    A 24V 200A system stores the same as a 12V 400A system. Still just 4, group 27 size cases.

    For loads, we need the amps and hours for your CPAP, TV1, TV2, Lights 1-5.
    Knowing how long each item draws current for, will give us the figure of what
    sort of PV system to size to, or if you want, we can figure out how much load you can use.

    Example: (volts X amps= watts)
    CPAP 3 amps 8 hours 3x12=36Watts x 8 hours = 288Watt hours

    Figure all your loads like this.

    Now your charging,
    120W panel flat on roof 4 hours of USABLE no-aim sun per day =
    120 x 4 = 480Watt hours Harvested / 2 = 240 Wh total usable by system.

    Yes, that 50% deduction is what the battery losses, panel losses and inverter losses ultimately add up to. Sucks don't it? Set your alarm clock to only sleep 7.7 hours instead of 8, and there you go - no generator usage.

    So to cheat, when you fire up the genset in the AM for hot muffins and chocolate
    you will be chargeing the batteries too, and then the solar can top it off.

    Hope this helps
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • coulter6coulter6 Registered Users Posts: 3
    Re: RV Solar System Design

    I appreciate the great info, allot of food for thought.

    From what I read (which makes me dangerous), the solar panels can be wired for 24v. They run through an MPPT controller (in this case the SB 3024iL) that converts the 24v solar to 12v battery charging. Am I mis-reading the info?

    Bob C.
  • BB.BB. Super Moderators, Administrators Posts: 33,313 admin
    Re: RV Solar System Design


    No, you got it correct... MPPT does efficiently convert higher voltage solar panel voltages into lower voltage battery charging.

    And, if you have long runs and/or a lot of panels, running the panels at a higher voltage will allow you to use smaller/cheaper copper cables between the array and the charge controller.

    Reserve the heavy cables for the short span between the controller and the battery bank.

    My own recommendation is that you try an limit your battery DC current input into the inverter to about 100 amps maximum... A 1,000 watts max at 12 volts, 2,000 watts max at 24v--and over that--use a 48 volt battery bank. Trying to manage >100amps of current gets expensive (lots of copper wire, big fuses/breakers, etc.).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • dwhdwh Solar Expert Posts: 1,341 ✭✭✭
    Re: RV Solar System Design
    coulter6 wrote: »
    4 6v house batteries for a total of 440amps

    Are these 220 amp hour batteries? If so, then shut my mouf. If they are T-105 type though, then they'll be a 220ah bank, *not* a 440ah.

    I.e., Trojan T-105 type 6v batteries provide 105 amp hours. Two of them, wired in series provides the same 105ah, but at 12v. Two pairs like that wired in parallel will double the amp hours to 220 at 12 volts.
  • coulter6coulter6 Registered Users Posts: 3
    Re: RV Solar System Design

    I appreciate all the responses and information.

    Here's what I know about wiring batteries:
    Rule 1. In series, voltage adds together, and amps are unchanged.
    Rule 2. In parallel, amps add together, and voltage is unchanged

    According to the Trojan Battery website the newer T105 6v batteries are rated 225 AH for 20hrs each.

    So according to rule #1, 2 T105 batteries wired in series would make it a 12v bank rated at 225 AH and applying rule #2, put this bank in parallel with another two 6v batteries (in series), and you have 12 Volts, and 450 AH.

    I plan on buying 4 batteries from Sams Club and I believe they are rated at 220 AH, so in my application I will have a battery bank that is 12v and 440 AH.

  • BB.BB. Super Moderators, Administrators Posts: 33,313 admin
    Re: RV Solar System Design


    Yep, you got it.

    And rule number 3:

    P = I*V = Watts (rate of energy usage/charging)
    E = AmpHours * Volts (Energy storage) = Watt*Hours

    Conservation of power, no matter how you wire the batteries, the stored energy remains the same.

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