Forum Discussion

chloe_s_ranch's avatar
chloe_s_ranch
Explorer II
Jan 31, 2014

solar options

Last summer I had a 240 watt kit(from Solar Blvd.) Installed on our MH. It has two 120 watt panels feeding a 20 amp charge controller. I recently had the two stock 12 volt marine house batteries replaced with four Trojan T-105s since we almost always dry camp or boondock and wanted more amp hours to get us through the shoulder season when we run the heater more(and there is less sunlight.) We also hate running the generator because of the noise. I know a rough rule of thumb is about 1 watt of solar for each amp hour of battery capacity and I now have a 450 amp hour battery bank. So if I have more solar installed here are some options: A--Have another kit from Solar Blvd.installed with its own wiring and another 20 amp charge controller so both kits would work in parallel charging the batteries. They have a 300 watt kit for $500. Or B--get their 120 watt kit and have them swap out the 20 amp charge controller for a Blue Sky sb3000i 30 amp mppt charge controller for about $500 also. I know option B would require heavier gage wiring since all three panels would be wired together feeding a single controller. Or option C--get the 300 watt kit and pay a little more to swap out the 20 amp pwm charge controller for a 40 amp pwm charge controller and wire all four panels into it. Again needing heavier gauge wiring. It seems to me that option A(having two separate kits feeding two separate 20 amp pwm charge controllers) offers the most bang for the buck. What are your thoughts?

41 Replies

Show More
  • BFL13's avatar
    BFL13
    Explorer II
    Jan 31, 2014
    First you need to figure the size of controller in amps needed for how many watts of panel. this is for PWM but you can go a bit more with some MPPTs where it clips the amps safely if you go over, where the PWMs let the amps through.

    You need 20% margin in controller amps over the panel rating for amps using Isc not Imp. So your two 120s should have Isc of 7.6a each and your rated amps is now 15.2 so 20% of that is another 3a so you need a controller rated for at least 18.2a so your 20a controller is correct.

    Working backwards, what can a 30a controller handle? the 30 is six parts and the panel five so the array can be 25a Isc or using 120w panels at 7.6a each that's three 120w panels.

    So if you plan to double your solar you will want a 40a controller (or two 20s)

    To choose which set-up is best depends a lot on whether you can do your own work I suppose. I agree two 20s and stay PWM is a good choice. Last summer I ran two arrays on my bank with each array having a 20. Total amps to the battery is the same with two 20s in parallel as using one big 40.

    However cost of controllers does not just double with amps, so one 40 can cost way more than two 20s. "Depends" on sales etc of course.

About Technical Issues

Having RV issues? Connect with others who have been in your shoes.24,331 PostsLatest Activity: Nov 04, 2025