I found a book and website that makes it simple.
HereI have but a single battery and like the OP use power lightly. The site I referenced recommended 200 watts for a single 100 AH flooded cell battery, 400 watts for two such batteries. I’ve just started using that system and it looks like it’ll keep my battery topped up easily even on partly to mostly cloudy days.
I think you’ll find 100 watts lacking if there are any clouds or your rig is shaded in your site (and don’t we find shady sites more pleasant in the summer heat)?