I was wondering with so many variables is there a rule of thumb on how large of a solar system you need to keep your batteries charged so not having to run the generator.The factors I would include is my trailer has LED lighting and a 40" TV. The TV may be used two or three hours a day... I have four group 24 12 volt batteries..Thanks for any input

One rule of thumb 100 watts solar for every 100 amp hours of battery. It really depends on your usage, you need to calculate the watts of each item you will be using for total watts/amp hours used and what is needed to put back into batteries.

A universal rule of thumb...NO.Do a simple spreadsheet that takes the wattage of each device that will be in use times an estimate of how long it will run to calculate the total watt-hr.Say you have a 75w TV runing for 4hours...300watt-hr.Say you have 5 - 20w light bulbs going for 3hr...300watt-hrSo if that's all you have, you need to replace 600watt-hr each day with solar. Usually you will get around 4hr at the rated output so figure 150watts of solar pannels (600/4 = 150)There are some catches that go along with that.- The last 10-20% charging your batteries goes much slower as they can't take a lot of power, so a ton of solar into a small battery bank can result in a lot of the solar being wasted.- With lead acid, you don't want to drop below 50% charge or you damage the batteries, so you need enough cushion to cover this. (to get battery amp-hr, divide the watt-hr by 12 to get amp-hr (12v). - Do you want to cover the occasional cloudy day? If you want the ability to go 3 cloudy days without resorting to the generator, you need to triple the watt-hr for storage and increase the solar so it can catch up when the sun comes back out.Nothing rocket science about the calculations. Make your best estimate and add a little cushion and you will do better than a random number thrown out.

How many watts you need ?,.....if its raining, or shining, or somewhere inbetween ??

First I would look at increasing battery capacity, either 4 GC-2 or another configuration depending on your carrying/storage capacity. How long do you want to go without charging from another source? A weekend trip or a 2 month stay? If your solar can get you back to 90% then you start the next day at 90% instead of 100% and you likely will end the next day at 80%. You cant blanket say, "this will do it" too many variables. Weather, daily usage, and they compound on each other. If you have a really rainy day, not only do you make less with solar, but you likely will use more from your batteries because you will watch more TV run more lights and other things more. One of the nice things about solar is that you can add to it. Start with 200-300 watts and a 40 amp controller, use it a while and see how well it covers your needs. Then, if it is insufficient, add one or two more panels as needed to reach your objective. A 40 amp controller should let you expand to fit your needs without buying a replacement.

You just need to calculate watts in versus watts out.When buying a panel go as big as possible. Anything less than 100 watts is a joke. The bigger the panel the cheaper it is on a watts/dollar basis. The best deals will be on panels in the 275 to 330 watt range. For these you will need an MPPT charge controller to match the output voltage to your battery bank. Don't skimp on the panel or the controller. Get a big panel, get a controller large enough to handle a few more panels. A 60 amp controller will handle 2 or maybe 3 of the large panels.

Solar Rule Of Thumb.... | Good Sam Community

mike-s
Explorer
Feb 08, 2018
OldSmokey wrote:
scrubjaysnest wrote:
HikeN2Mts wrote:
One rule of thumb 100 watts solar for every 100 amp hours of battery. It really depends on your usage, you need to calculate the watts of each item you will be using for total watts/amp hours used and what is needed to put back into batteries.

While the above is the universal rule of thumb; we find in practice 140 watts per 100 aH is a better choice.

please don't post arbitrary nonsense like this, all it will do is result in frustration and failure for those that adopt it.
My partners and I just shook our heads this morning.. let me give you an example of why there is no rule of thumb that will work.

my rv has 960 watts of solar.. so how many a/h do i have ?
can u guess ? no, there is insufficient data. whats missing ?..
well two things. 1) Voltage 2) Autonomy
change any one of those two and your thumb is blown away.

i do, in fact have a 300 a/h 48 Volt bank of prizmatics
that new thumb ratio for you is now 3.2:1.

i dont wish to sound arrogant, but do you understand now why
a 1:1 rule is just bull ?

doing it right is very simple, no thumbs required!
The GP (and the common 1W:1Ah rule of thumb) make the assumption of a "12 V" system, which applies 99+% of the time for an RV, and reasonably applies to the OP's rig, which this discussion is about.

Translating to those terms, you have a 960/(300*4)= 0.8:1 ratio system, close enough to the 1:1 rule of thumb that your system doesn't make it "bull," but supports that rule-of-thumb.
full_mosey
Explorer
Feb 09, 2018
OldSmokey wrote:

...
i do, in fact have a 300 a/h 48 Volt bank of prizmatics
that new thumb ratio for you is now 3.2:1.

i dont wish to sound arrogant, but do you understand now why
a 1:1 rule is just bull ?

doing it right is very simple, no thumbs required!

Nice try!

That 1W:1Ah ratio is based on a 12V battery bank.

Adjusting your bank to a 12V basis gives you a 1200Ah bank. Your ratio is 0.8W:1A.

And there we are right back at a little under 1W:1Ah. :)

HTH;
John
time2roll
Nomad
Feb 09, 2018
OldSmokey wrote:
do you have a monitor ? i bet u do..

It is in my signature. I spent my monitor money on more batteries.
Monitor does not add amp hours or any functionality for me.

And for all you techies, a pic of my battery monitor.
mike-s
Explorer
Feb 09, 2018
time2roll wrote:
I spent my monitor money on more batteries.
Good for you. Your monitor is your brain. An electronic "monitor" is an AR thing in any reasonable system.

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