I have also wrestled with the figures in the past. But it depends on so many variables that I got bored. So here is what I am finding in the real world with my 80w set up.
I run the following from a secondry battery in my Discovery.
Car CD/Radio
CB
Engel 45 Ltr Fridge
2 x Permanently fitted LED work lights for cooking etc.
1 X Portable Fluro light (double tube)
Laptop
Charging camera / phone etc
When the sun is out (even a little) all of the above can be running and the regulator is still charging the battery. Obviously I don't need the lights during the day
but was just testing the set up. When the battery gets to about 13.8v the regulator stops charging the battery till it drops to about 12.3 then flashes up again till it reaches 13.8v bla bla bla. The fridge is on 24/7 and I have also left all 3 lights on overnight to see if the battery can cope....and it does.
My secondry battery is a standard halfords battery that is a direct replacement for my existing battery. That way I can always replace the main battery if required.
Although my 80w panel is strapped to the roof rack 90% of the time, it can be removed and used as a portable unit. The regulator switches itself off whilst driving as the 2nd battery is being charged via a split charge system. Because it is portable on a stand, I can park under in the shade under a tree etc and put the panel in the sun using a 5m extension. Never needed to do so though.
Basically I find that I can camp 24/7 without having to use the alternator to charge the battery. I'm off to France in a few weeks so will let you know how it performs.
Couple of photo's: