Solar Rule Of Thumb....

[1L243]

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

[KD4UPL]

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.

[kerrlakeRoo]

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.

[RJsfishin]

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

[valhalla360]

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-hr

So 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.

[HikeN2Mts]

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.