If you are going to use solar power there are a number of things to consider. The bigger the panel, the more amps it can pass to the controller to pass to the batteries. A 100 watt panel in good sun can do around 5 amps per hour charge to the batteries. A rule of the thumb is the battery or batteries should be charged at about 10% of their Amp Hour rating.
An Amp Hour rating for a car sized deep cycle battery is about 80 Amp Hours. A good charging amperage would be about 8 amps. So this 100 watt panel can come close to a decent charge to that battery. If you have a pair of Trojan T-105's in there, they are rated at 225AH for the pair. This means about a 25 amp charge to them is best. So that 100 watt panel will indeed apply a charge to the batteries, but not enough on it's own to actually give them a good charge. It is still better than nothing, and a great supplement to your charging system.
The 100 watt panel operates at about 17 volts. The solar charge controller regulates that down to around 14.5 volts to charge your 12 volt batteries. A very basic on/off switch type $30 controller does a very basic job of attempting to allow juice through or not to put a charge into those batteries.
A $100 PWM ( Pulse Width Modulation ) solar charge controller lets you have more control over how that battery is being charged. It is a good model for a simple system. They are generally preset with the manufacture's best estimate at charging rates for the three stages of charging the battery needs to live the longest.
A $200 MPPT ( Maximum Power Point Tracking ) solar charge controller lets you go to town on doing things right. You can set it to what voltage you want it to be for each of the 3 stages of charging and with the optional display panel (for around $75), this bad boy lets you see in the display exactly what amperage is going in, and exactly what amperage is going out. It also lets you see how much battery life is remaining at a glance.
All charge controllers are rated in amps. From a crappy little 7 amp one, up to big monsters that do everything extremely well. A decent 20 amp controller lets you cover all your bases for average camping and gives you the option of adding more solar panels down the road if you want.
So much more than you might think goes into charging a battery. It even matters what brand battery it is. Trojan engineers want their batteries charged at slightly different voltages than the other guys for the longest life. Your charge controller factory defaults are not likely to be those exact voltages. Being able to program into your display / program panel lets YOU decide what the charge controller will do and for how long.
What type of battery it is matters too. A lead acid and an AGM battery have different charge rates. Things like even the temperature matter. You know your car is harder to start when it is cold. So if your batteries are sitting in 5 degrees compared to 25 degrees, they will need to recharge at a different voltage / amperage rates.
It isn't really. A decent quality panel, a decent programmable charge controller, and decent Trojan batteries will have you going off grid quite well.
The Inverter you use to draw the power out also has lot's of things to consider too, and we can help you with your choices of how those batteries are used efficiently to draw the power out, in the same way we can show you how to draw the power in from the solar panel.
For charging tips, read our other blogs, and Happy Camping.