I looked at a 6KW home system around 15 months ago. The short/simple answer is that the direct costs of such a system was not cheap - it was somewhere around $40K before the local (State - NJ) rebate of roughly 50%.

When you then take that capital outlay and divide by the monthly savings in electricity, it works out to around a 20 year time-until-payback in KISS form. Obviously unacceptable ROI, but more curiously, way, way off from what the claims were.

What I found was that (at least locally), the owner of the PV system has the ability to register at a State website for their power generation and effectively what happens is that as they generate power, the PV owner sells off their "Green" (Renewable Energy) energy credits, and it is this revenue that dramatically changes the ROI numbers.

These 'Green' sales are being sold to the local Utility company who is required by regulation to have some small percentage of their total power generation be from renewable resources. That's not necessarily a good or bad thing - the main thing is to know where the money's coming from and the general makeup of the market in order to assess your longer term prospects of that marketplace remaining effectively the same over 2, 5, 7, 10 years ... ie, until you at least hit your ROI.

However, this sale is done as an auction, so very simplistically, with more people installing PV, the supply's going to increase, and since the demand is artificial, it seems to me to be unlikely that the claimed ROI's are going to hold up in reality over the needed longer term.

-hh