For anyone who is asking the question (or is concerned with) about solar, batteries, charging, etc....a Battery Monitor is a MUST (...and an amazing/fun/informative tool).
Here is an inexpensive one for less than $50:
https://www.amazon.com/gp/product/B07FGFFHC6
If you want to get fancier, there are some with bluetooth that connect to your phone apps, etc.
Once you install a battery monitor, go out for a weekend trip and live life as you normally would when RVing. Write down data points from your battery monitor...Start the batteries fully charged (100%). How low do your batteries go after 24hrs? 48hrs? This will give you an idea as to how many total AHs you "usually" consume each day. From there, you can calculate how large your battery bank should be. It is reasonable to double that amount of AHs (...this will give you some buffer in your overall AHs). If your reliance on batteries is CRITICAL (ie. If you MUST have batteries for work, etc) then add more. And then from there you can calculate how much solar you need to get your batteries charged near full everyday. If you have a generator, you always have a backup. If your batteries charge from your chassis alternator then you have another backup.
If you live fulltime (or more than 50-100 nights per year) then battery cycles/lifetime becomes an issue and you might want to look at the DOD vs lifecycle charts for your specific battery and design your system accordingly. BUT if you are spending less than 30 nights per year BOONDOCKING in your RV then you can pretty much ignore the "don't discharge your batteries more than 50%" mantra that you read online (...you will never "use" all your battery cycles even if you discharge down to ~75-80% every time you camp...as long as you charge the batteries back up the next day). I'm not saying you should assume 75-80% discharge when designing your system (...I like to be conservative and don't mind if I have more batteries than I need), but I'm saying you should never worry if your batteries go that low during use if you are like most non-fulltime RV users.
If you have four 6v GC batteries, then your current battery bank AHs is 400-450AHs. If this were me, I would try to design my solar so that it could replenish ~250AHs per day. One 100w panel on average during summer with good sun overhead will generate ~30AHs/day. So 900w could probably generate ~270AHs per day. Because I like to overdesign everything, I'd probably put up 4 of those 300w panels and that would be plenty.
One thing you need to keep in mind that many people don't account for... When your batteries are discharged 40-75% they will accept a LOT of amps from your charge controller. BUT once your batteries get to about 80% full the charge rate from your charge controller starts to slow down SIGNIFICANTLY. Even if your 1200w of solar panels are generating 60 amps that *could* be flowing into your batteries, you might only see 2-10 amps actually flowing into your batteries. That's because that is all the batteries want (or can handle) during that stage of the charge.
One other good news/bad news thing to consider....
Bad news: IF you are using tons of power at night, then all of it is coming from your batteries as there is no solar production at night.
Good news:
1) Generally we sleep at night, so power draw is usually not crazy while we are sleeping (...the furnace on cold nights likely being among the largest power draw).
2) If you are using lots of power during the day (ie. Your Starlink, working computers, etc), your over-designed solar system will not only charge your batteries at the quickest rate that the batteries will allow, but the EXTRA power coming from your solar panels will power everything in your RV/trailer that is using power during the day. So the reality is that you will have "fully charged" (~98% full as the last 2-3% take a long time) batteries at dusk. So your battery bank really only needs to get you from dusk to ~8am when the sun comes back up (and the solar starts to kick in again)!
With solar, the reality is you need a smaller battery bank than you think to get you from dusk to dawn. BUT, since you can't always count on sunshine, calculating your actual typical daily AHs needed and then doubling it seems to be a good technique for calculating how big your battery bank should be. And, if you have a generator and alternator as backup, then you are completely covered anyways.
Other random points:
If you are camping in the heat and you run your generator during the day so that you can run your AC, then solar isn't needed as much since your generator will be charging your batteries.
If you mostly camp with electrical hookups, then you don't really need solar.
I like having at least SOME solar so that my coach and chassis batteries (via AMP-L-Start) are always kept fully charged especially when in storage for weeks/months.
I like using used solar panels on my RVs. I have bought many 250w (working/tested) solar panels for $35 per panel. No moving parts means they last a very long time. At that price, throw up an extra panel (overdesign your system) and call it done.
Good luck!
Chris
PS: Solar is addictive
San Jose, CA
Own two 2015 Thor Majestic 28a Class C RVs
