Actually not a great idea. Grid tie panels are different. You really need to make the decision right up front. For example if you want to run 12v DC stuff like lights and fans and small appliances, and have some inverters for AC stuff, you'll be buying panels that put out 17-18v to charge 12v batteries. On the other hand, if you want to do grid tie you'll be buying panels that put out 23v or so.
I found this out when I got all excited about 200watt panels I wanted for my 12v system. If the watts are that high, they are probably for grid-tie systems. The biggest I could find (and I'd love to hear from an Alt-E person if this has changed) is 125-130watts for a 12v system.
I've wrestled with this same question and have concluded that if you want to do grid tie, just plan on taking out a big loan and do it all at once. Make sure you do your math carefully on how long your payback will be in relation to utility savings.
If you want to grow a small system into a bigger one like I do, you can do it a few bucks at a time. I'm attacking my ranch in modules and don't feel a need to even have the whole place on one single unified system. I will stay on the grid, but I don't want my system grid-tied cuz our grid power goes down 3-4x a year.
You can buy more panels as you can afford them, but if you're growing a battery bank try to complete it in a few months so you don't end up with batteries of different ages. Also, a huge charge controller like a Xantrex c-60 is not that much more than smaller 30 amp controller, so start with the biggest charge controller you can imagine ever needing.
As your system grows beyond that, you can have entirely different charge controllers and battery banks to do different duties in the home.
That's my perspective on the piece-meal plan. If you want a unified solar system that runs everything, get a loan and hire someone smart.
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