I've made the numbers match (i.e., same cost for PV installation or using power from grid, assuming defensive increases in cost of electricity for the next 15 years - only going up as much as inflation) with a slightly higher subsidy (1000€ subsidy on an installation cost of a little under 12k€, which would only buy an installation that doesn't produce 100% of an average consumption).
But when the costs converge this closely then small assumptions can tip the scale one way or the other - one % extra opportunity cost, a converter that needs replacing 5 years earlier, 10% less sunshine in one of the earlier years of the installation's lifespan, ... That said though, when you extrapolate costs of grid electricity of the last 10 years, and base your energy price scenario on that (instead of keeping up with inflation like I did), PV comes out ahead even when taking worst-case scenarios of all other variables.
Many people argue that because prices are falling so fast, it doesn't make sense to buy now. But IMO it's much more nuanced than that:
- As prices fall, so will subsidies. Already most subsidies are being phased out or decreased.
- There is a floor on the prices - in the short term that floor is just below grid parity, in the medium and longer term it will be production limits. I don't think it's rational to assume that prices will keep falling so fast.
Many words have been shed on this - I guess only time will tell. That said, like I posted elsewhere here, my parents are getting an ROI of 11% on their panels. That's mighty good for money that can be borrowed under mortgage, and 'green' == cheap mortgages for that matter. (that yield is only because of subsidies though, so it's not really a fair general case comparison).