Join Date: Apr 2006
Thanked 162 Times in 159 Posts
Rep Power: 12
calm, in order to keep the batteries topped up when not in use you don't need much power. Something likely 30mAH per battery per day, so one or two (keeping things separate, one to each battery, is a simple robust way to go) small 10W panels would do that job.
Keeping the batteries charged when you are using the boat is something entirely different. Yes, an energy budget is the right way to do it. But in practice I've never heard of anyone being able to afford more panel than they could really use, and following that logic you just buy as much panel as you can reasonably install on the boat. On a 30' boat that might be one or two 2x3 panels. Finding a place to install it (or them) where it won't be shaded or stepped on is the big trick.
At that point you work the other way: Buy the panel you can afford and accomodate, then deal with that much power as being all you're going to get. Switching to LED lighting and running lights (if you sail much at night) can be a very cost-effective way to stretch your power budget, despite the high cost of them. Cheaper than adding power sources, fuel, or batteries.