The holy grail -- grid tie sunny boys and preparing for when the grid dies...

I'm in the process of installing a 12.5kw grid tie system at a vacation home. It will use a pair of sunny boys, and I also want to figure out how to have the system work if the grid goes away for weeks at a time. Most of us are interested in this capability. It is a little like everyone wanting a 300HP diesel aircraft engine. They are not really available.

So I had an idea, if you have sunny boys, then the answer is to have a couple of sunny islands and a big pile of batteries. I am informed that this solution works. Of course it is cheaper to buy a two years' supply of propane and a generator, which is the problem. SMA tells me that for this purpose, you can use a very small battery backup, particularly if you are not interested in powering your home unless the sun is shining. Fine with me! So the main expense (assuming we can mitigate the battery expense) is that you need a pair of sunny islands. What if you were to buy a big 120/240 transformer (probably about 1/5 the price of a second sunny island), is this possible? Then a single sunny island could talk to the sunny boys, right? Or have I been talking to Mr. Beam too much?

Thanks