Newbie needs help with a new system design

I inherited a old Class B RV that included parts to put together a small solar power system for off line power augmentation. There are three BP350 50watt panels, a Morningstar Sunsaver-10 and a Morningstar Sunsaver-5. That's it - no plans, no wire, no connectors. 

Right now I do not want to do a roof mount, but rather have a ground system that I can move around and plug in. I think I want something I can use to charge the "house" battery, the "car" battery, and maybe the trolling motor battery on occasion. My thought was to use a couple of 20amp cigarette plugs for the "in vehicle" connections, and maybe simple clips for the trolling motor

I am looking for help with everything from wire size and length, fuse/ breakers needed, and connectors to make it all simple to put together. I think I know that the controller should be close to the battery end of the wire. The panels each have a diode in the connection box, not sure why. I am comfortable wiring in the vehicle with a diagram but my 12V integrated RV system design knowledge is somewhat limited



  • BB.BB. Super Moderators, Administrators Posts: 31,685 admin
    Welcome to the forum Stephen,

    Is your question how to understand/update as needed the existing system, or do you want to design and and install a new/more capable system?

    3x 50 Watt panels are pretty small and obsolete these days--Unless you need very little energy and have a relatively small battery bank.

    These days with wanting to run tablet/laptop computer, LED lighting, cell phone charging, drone/camera charging, most people want more energy (probably in the 300 to 500+ Watt solar array vs the 150 Watt array you currently have).

    Your energy usage is highly personal set of choices--So understanding your needs is the first place to start--And design a balanced system around that...

    However, if 150 Watt array is what you need--A "balanced system design" would be something like this:
    • 150 Watt array * 0.77 panel+controller derating * 1/17.5 volt Vmp = 6.6 Amps @ 12 volts from array (for PWM charge controller)
    • 6.6 amps * 1/0.05 rate of charge = 132 AH @ 12 volt max suggested battery for off grid usage
    • 6.6 amps * 1/0.10 rate of charge = 66 AH @ 12 volt "nominal" battery for full time off grid
    • 6.6 amps * 1/0.13 rate of charge = 77 AH @ 12 volt "suggested minimum" battery for off grid
    How much energy you can harvest depends on where you are located and the tilt of your panels. For example Washington DC, south facing array, tilted to face noon time sun (more vertical in winter, flatter in summer):

    Average Solar Insolation figures

    Measured in kWh/m2/day onto a solar panel where the angle is adjusted each month to get optimum sunlight.
    Jan Feb Mar Apr May Jun
    Jul Aug Sep Oct Nov Dec
    And there is the harvest if you mount flat to roof (such as on an RV):

    Average Solar Insolation figures

    Measured in kWh/m2/day onto a horizontal surface:
    Jan Feb Mar Apr May Jun
    Jul Aug Sep Oct Nov Dec
    Just to give you an idea of the math... Say 4.5 hours of sun (summer camping) (typical charge during day, use 12 VDC power at night):
    • 150 Watts array * 0.61 panel+controller derating * 4.5 hours of sun per day = 412 Watt*Hours per day
    • 412 Watt*hours per day * 1/12 volts = 34 AH @ 12 hours per average summer day
    You should plan on using about 1/4 to 1/2 of your battery's storage capacity per day:
    • 66 AH * 1/4 capacity = 16.5 AH over night
    • 66 AH * 12 volts * 1/4 capacity = 198 WH over night
    For a typical off grid cabin/home, would suggest 25% per day and 50% max discharge for planning (fully charge next day, longer battery life). For an RV weekend use, can use 50% max per day and 50% max discharge as battery will probably AGE out before WEARING out.

    The above numbers are based on using a flooded cell lead acid battery bank... But are "close enough" to work for a first cut at a Lithium Ion (LeFePO4) battery bank.

    Regarding setting panels on the ground... Yes, it is nice you can park in shade and place panels in sun... However, there are issues. 1) Theft; 2) Need to stake to ground to prevent wind blowing over and shattering glass; 3) Have to pack panels when moving trailer, only charging with solar when panels are deployed.

    Roof mounting is usually nicer... But if you do winter camping in the North, then tilting the array to catch the winter sun can help quite a bit. However, solar panels only work in direct sun--Bad winter weather, shading from trees/deep valleys, etc.--No direct sun, no solar power.

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