Absolutely agree with the baby steps and educating self along the way. I also agree this is a difficult question to consider without more of the items you're using and how often or in what series. My suggestion is consider your consumption and develop for twice that. a great reserve is important but then so too is ensuring you have the right bells and whistles in place when wiring, battery banks, etc.
I guess a good place to start, would be to consider what you are running, what you're left with at the end of the day and how much you need to replace during the next . voltmeter and understanding the power drained from your bank while using. self moderation some would say... do the math, and again plan for at least double. not hard to do.
