Calculating alternator requirements

I'm OK with mechanical stuff, but electrical is just to the left of witchcraft if you ask me. So, how do I go about figuring out how large an alternator I need, based on the accessories I have now and what I may do in the future. I have a relatively new 60 amp alternator and have added no electrical accessories as of yet. I may look at things like electric fan, electric water pump, stereo, etc., and want to make sure my current alternator is up to snuff, and/or upgrade when necessary. I just have no idea how to know how much is enough, and don't really want to go buy a bigger alternator if I don't need to.