If you have LED lighting in your cabin, with all the lights on from dark until bed time, you'll probably not burn more than one amp. If they're 12v lights, lets say 3-4 amps. I'm not sure how much power your auto pilot uses, but for a 26ft boat with a tiller, I would bet maybe 2-3 amps. For wheel steering, a couple more. (That is a complete guess on my part) If you're jamming out on the radio all day during your sail, a 25w head unit (typical aftermarket) uses 2 amps.
Basically, you need to add up all your load (how many amps from each device you're using), multiply by how many hours that device will be operating, give yourself several amps of wiggle room, and that's your total amp hours needed for a day. For your stereo, 2amps times 8 hours = 16amps. You'll probably not use more than 4 amps all night for your lights if they bulbs are 12v. So that's 20amps. So a 100ah battery might come down to only 80 amps. Maybe 70 or so with an auto pilot.
The average 60w panel will probably produce a max of 3.5amps. When looking at panels, pay close attention to how many amps the panel will produce. This number (let's say 3.5amps) x 8 hours of sun a day will charge 28 amps. So if your total usage was 20-30 amps and your 60w panel produces 28 amps, then that's all you need. Any excess power produced will be released your charge controller in the form of heat. It is your charge controller's job to regulate the power your PV array (panels) deliver to the battery. It allows the battery to not be over charged.
One 50-60w panel, which shouldn't cost more than about $70-75, one charge controller (Maybe a 7amp controller... it is good for your controller to be able to handle twice the amount of power as your PV array puts out so it doesn't get over worked), and you're set. To allow for bad weather, just add another battery or two. Your reserve power (the amount of amp hours in your batteries) has more to do with your needs than any other part of the system. Even a very small solar panel can keep your batteries topped off for part timers and weekenders if the battery bank is large. With three 100ah batteries (300ah), if you had cloudy/rainy weather for four days straight and still used 25 amps a day (4 days x 25amps = 100 amps), you would not deplete your batteries below 66%. A good battery bank allows for bad weather. Even just adding one doubles the amount of power you can use before needing to be topped off. Of course, the batteries don't all have to be the same size. One 100ah battery and one 150ah battery obviously gives you 250ah.
In short, for your current needs, a 100w panel would be fine, but you really don't need one that big, even if you decide to add a fridge. A 50-60w panel, a 7 or 8 amp charge controller, and one more battery is all you should need I think. Most panels under 100w can be had for $1.30 a watt or less. Larger panels can be bought for under $1 a watt. Check out solarblvd.com... Good prices, lots of choices, educational articles... I love that site. You can always grab a calculator and do some simple math to see how your needs are changing. Watts = Volts x Amps. 36W = 12V x 3A. 3A = 36W / 12V, etc.
Good luck mate!
