SolarCity was described by more than one person as a mortgage company that happened to sell solar panels. The advantage of TPO systems is that you can get into them for nothing. The disadvantage is you're swapping one utility for another, usually with a 20-year commitment.
First Solar chased Solar City out of Nevada by arguing that they weren't entitled to the individual solar credit under state law because they weren't individual homeowners. Solar City argued that yeah, they kinda were, after all they had all these liens. The court was not amused. Tesla bought out Solar City about nine months later. First Solar, for their part, sells to utilities - they aren't interested in rolling up a million mortgages when they can put acres out in the desert. Solar panels in Nevada are something like eight times as useful as solar panels in Seattle for obvious reasons... and if you're going for efficiency, why not put the panels out where the sun is?