2 watt or 5, it's not enough to do anything but keep it topped off. You will need at least 15 to charge, and even then, you will need to be conservative in use.
The best way to make solar
the most efficient is NOT to add more and more power, but to set yourself up to USE less. And I don't mean you have to go without, or work more than needed to perform simple tasks, or resort to using the old car camping stuff. I am in my trailer right now with a tv going, the computer and almost ALL my lights
on. The heater is on low as well. My Soda is cold, my water flows freely and I am looking forward to LOTS of coffee in the am.. without worry of draining the battery.
I am charging with 45 watts for the house battery. It was overcast all day. When the sun went down, I was sitting at 13.2 volts. All this stuff went on, now I am at 12.8 after 2 1/2 hours.
I am working on a web page that has all my systems on it, when it is done, I will post a link. It's really astounding what you can do with solar
if you just look at usage a little differently.
A 15 watter should do OK to get an evenings charge. If you want to get more, the beauty of solar is it is "add onable".