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?

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.

A -- would seem the easiest but it would be nice to have controllers that could talk to each other. With B, the SB3000i will not work well with over 400 watts of solar panels, I'd step up to the SB3024iL and an IPN-ProRemote. With A and C, the 150 watt panels with the 300 watt kit should have voltage outputs within a volt or less than your current 120 watt panels.I would just buy two panels exactly like the panels you already have and a 40 amp controller, and rewire with #4 from a combiner box on top down to the controller. Do you have a battery monitor?

First I would review the current system on how it works for you. The additional battery may extract more power from the existing solar and get you through just fine. Also look for shading issues with the existing system. Even part shade on one cell can cause that whole panel to drop out. Get up on the roof at 9am and again at 4pm to observe.If you are thinking MPPT I would buy like panels and run all four in series with one loop of #10 MC4 cables down to a 45 amp Morningstar controller.

Hi,I would be inclined to just wait and see if what you have doesn't work out fine. I suspect that it will. If you don't already have one, I would recommend a battery monitor (like this)Steve

I would be sort of inclined to consider SteveAE's opinion; but think of having the two panels rewired in series, and changing to an MPPT controller that would handle 40 Amps for future growth if it becomes necessary.The MPPT controller would give some added useful output from the panels that you already have and that your new bigger battery could utilize.Doug

solar options | Good Sam Community

41 Replies

pianotuna
Nomad III
Feb 04, 2014
Hi chloe,

If you are using a pwm controller voltage drop doesn't matter, so long as the wire is "legal" for the number of amps of current.

example:

17 volt panel drops down to 14.8 volts by the pwm controller. That is about a 13% voltage drop. Quite a bit more than you would ever see from the #10 wire.

If you are using an MPPT controller, place the panels in series--just make sure that nothing on the roof can cause shading on any of the panels at any time.

OR

Replace the #10 wire, use MPPT and a parallel panel configuration.

If you won't be going over 480 watts of panels, then the 3048 Rogue is state of the art, and includes a temperature sensor. Temperature compensation is a "must do" in my opinion.

Blue Sky is.....some what out of date and charge for extra for the temperature sensor. They also do not have battery voltage sense wires.

chloe's ranch wrote:
Ed suggested using a second 10 guage wire between controller and batteries. Is that better than using a single heavier guage wire? Easier since one 10 guage wire is already in use? Two tens equals what guage single wire? The controller is about three feet from the batteries. Thanks again!
NinerBikes
Explorer
Feb 04, 2014
10 gauge for 30 amps, or under, from the panel to the controller, will work. You'll want something like 6 or 8 gauge from the solar panel charge controller directly to the battery terminals, and as short a run as possible. The loses between the charge controller and the battery pack, after the volts and amps are controlled, is the critical wire diameter and lenght issus. Keep them short, and fat, to minimize losses in voltage and amps. Fat wire is low resistance wire.
time2roll
Nomad
Feb 04, 2014
Don't use parallel wires. Get the right stuff.
NEC and RVIA frown on parallel.
chloe_s_ranch
Explorer
Feb 04, 2014
Ed suggested using a second 10 guage wire between controller and batteries. Is that better than using a single heavier guage wire? Easier since one 10 guage wire is already in use? Two tens equals what guage single wire? The controller is about three feet from the batteries. Thanks again!
westend
Explorer
Feb 04, 2014
It sounds like you will definitely be upgrading the amount of panel you have and connectivity is at question. If you use a combiner box on the roof, it will all be a lot easier, no matter which wiring schedule you choose. The combiner box can offer easy termination of parallel sources and can also be used for a series string.

Since I'm a miser about lost voltage and I already had a spool of 4 AWG , I used that from the combiner box to the controller. I only have one module, at present, but can easily add more, if needed. Like Niner says, initial tally of how much power used in a typical day (along with some headroom) is a good starting point.

At some point, you may want to think about multiple higher voltage modules. There's some cost and ease of use benefits with the bigger modules. The layout of your roof will dictate, sometimes, what you can fit. The higher voltage panels will also mean MPPT controller, only.
FWIW, I have a Morningstar controller and it is quite a sophisticated device for the price. It allows a user to tailor charge settings, duration, equalization, and is temp compensated. It also has a PC interface so data logging, graphs, etc. can be made and saved. Lots of geekiness.

Good luck with your upgrade.
eb145
Explorer II
Feb 04, 2014
chloe's ranch wrote:
Thanks again everyone for the help. Sounds like an adjustable controller might be a good idea. If I do go with option B(adding a third 120 watt panel to my current 240 watt system and using the Blue Sky 30 amp mppt controller) I have another question. Currently my two 120 watt panels are wired in parallel with 10 guage wire down a vent to the existing controller. Should I have the additional 120 watt panel run with a separate 10 guage wire down the vent and then spliced together with the current wire at the controller, and then 6 or 8 guage wire from controller to batteries?

A single 10 gauge wire can handle the 3 120 watt panels in parallel. Yes there will be some voltage drop, but that is usually not the critical voltage drop for your system. The controller (PWM or MPPT) will reduce the voltage even more as it does its work.

What is critical is the voltage drop from your controller to batteries - that is what can really slow down the charging. Short fat wire is needed from the controller to the battery to minimize voltage drop between them. What gauge wire is currently between your battery and controller? And how long is the wire run between them?

A great way to measure the voltage drop between controller and battery is to measure the voltage across the controller output and then across the battery posts when the solar panels are sending lots of amps into your batteries. The voltage at the battery terminals is what needs to be high to get the most amps into the batteries.

So while adding a 10 gauge wire from your new solar panel to your controller will help a little bit, it is more likely that adding that 10 gauge wire between your batteries and controller will provide you with more benefit. Note I said "adding" the wire between batteries and controller - you can add more wires between the two points or substitute thicker pure copper wires between them - whatever it takes to minimize voltage drop at the battery posts.

Ed
BFL13
Explorer II
Feb 03, 2014
With the MPPT you could put the three 120s in a series string and stay with #10/8 Assumes the controller can take over 66Voc
NinerBikes
Explorer
Feb 03, 2014
double post
NinerBikes
Explorer
Feb 03, 2014
With four T-105's, you have 450 Amp hours at 12v. 50% discharge means that you'd need to replace 225 amp hours, per day. Basic rule of thumb, on the weak side, is 1 watt of solar, or more, per amp hour of battery. In the winter, you will likely need more than 1 watt per amp hour.

Since you already have 2 120 watt panels, for 240 watts, and about 13 amps, I would add another 2 120 watt panels, if you have the room, at a minimum, which should bring you up to a max charge rate of 26 amps an hour. You are capable of using 250 amps a day, that's 11 hours of charging under ideal conditions to get it all back. In winter, 6 or 7 hours is tops. Each panels will generate about 6. to 6.5 amps, peak, so a 30 amp converter of some sort should handle all your needs, in the summer, not so much in the winter, more panels are needed due to lower sun angle and shorter days.

Something like this:

Charge controller

In my view, until you know what your daily consumption is in amp hours per day, it's pointless to buy more panels, one at a time, or two at a time.

Nail down how many amp hours per day you go through on the shortest day of the year, and take it from there for worst case scenario.
chloe_s_ranch
Explorer
Feb 03, 2014
Thanks again everyone for the help. Sounds like an adjustable controller might be a good idea. If I do go with option B(adding a third 120 watt panel to my current 240 watt system and using the Blue Sky 30 amp mppt controller) I have another question. Currently my two 120 watt panels are wired in parallel with 10 guage wire down a vent to the existing controller. Should I have the additional 120 watt panel run with a separate 10 guage wire down the vent and then spliced together with the current wire at the controller, and then 6 or 8 guage wire from controller to batteries?

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