It makes sense for two reasons: firstly there's limited economies of scale in solar panels: they more or less make the same amount of energy per unit area and p

It makes sense for two reasons: firstly there's limited economies of scale in solar panels: they more or less make the same amount of energy per unit area and per dollar regardless of the size of the installation (rooftop is usually more expensive for installation but these systems have near zero installation cost). Secondly, because the generation is local you're not paying for any margin, co-ordination costs, or transmission costs. This secondary effect is already enough to offset the increased cost of rooftop installation in many cases. (And maybe you can consider this a market failure, in that it doesn't fit some naïve idea of an efficient market, but you're always going to be paying for someone to make and take on the risk of the big, centralised version of things)