We just signed up for almost 8KW on the garage roof. It's justified based on replacing our propane furnace with a heat pump that also gives us aIr conditioning, and also running an electric car (e.g. Nissan Leaf). We can use our existing monitoring system, but unlike last time, when we bought the system, we plan to lease it from Solar City this time. There Is a four month lead time, so we can pick the exact terms of the lease next February before it is installed. There are several options, zero down with a monthly charge that increases slightly each year, or options to pay various amounts up front with a fixed monthly payment. The monthly payments are all less than a typical electricity bill.
The goal for next year is to only use propane to run the backup generator (usually for a few days a year), and to make a dent in our petrol usage for the daily commute. We are on the Nissan Leaf list, but not in the first wave of owners. We should be able to get a test drive soon, I will report on any progress as it happens.
If you are interested in electric cars, look for Robert Llewellyn's Fully Charged video show on YouTube. He has been test driving everything and he's good fun to watch. You may recognize him as the actor who played Kryton in Red Dwarf.