It depends on your electric needs, obviously.
I run on a 15 watts panel. It works adequately, even on an overcast day, to run my lights
and tv at nite. I have run over a week on strictly solar
charging starting out with a FULLY charged group 24 battery
I have just about everything BUT lights
that run either manually (Water pump) or on propane
(Fridge and heat) The heater runs a little fan. My lights
are low draw florescants and I use them conservatively. I also avoid using my ceiling fan except when absolutely necessary for cooking etc.
If you have a bigger TV or other elctrical needs, than 15 probably will not due for you. Just HOW much you need, as mentioned, depends on what you use.
I want to expand with another 15 watt panel, as it gives me a little extra edge, but I think with the two, for just me, it will be more than enough.
Remember, these panels will not bring a dead battery back to life quickly. Sometimes it could take days. Always start out with a fresh full charge and depend on the solar to replenish.
To your left is a link for solar tips Solar Linky