There are all kinds of formulas for figuring that out, but I don't know them off the top of my head. Just know that a 100 watt panel is rated 100 watts in a lab, not the real world. So a 100 watt panel at sea level is more like an 85 watt panel. Then controller losses, etc. I prefer to over panel if possible. See how much panel your controller can handle and go with that - is my style. Then you could always add a second battery in parallel to double your capacity, but enough panel will be even more important.
It's a long learning process. I am just now feeling like I have improved from totally clueless to know enough to be dangerous.
