The idea of pushing for OLD homes to have solar added is a mistake. In addition, paying subsidies for it, is just plain wrong since it is causing solar companies to focus on just those locations.
Instead, at a US national level, we should put in place several regs and 1 new subsidy, while removing all of the other subsidies for Solar:
1) require that ALL utilities to buy up to 10% of a buildings excess electricity that is generated via on-site AE. IOW, if a building is expected to USE 1MW / month, then .1MW / month can be sold to the utility. In addition, it needs to be bought at the top that the utilitity pays for that time, to any other provider, including buying it from other providers.
2) require that ALL new buildings of 5 stories and less to have enough on-site AE to equal the HVAC energy needs (and require heating and cooling). Note that such a building with only enough on-site to equal the HVAC will likely not be selling much if any to the utility. However, if they decide to increase it, to the point that they equal 110% of their energy needs, then the utility must buy the extra 10%. Note that the smart developers will focus on lower energy costs buildings with better insulation and hopefully geo-thermal HVAC, since all forms of AE is actually expensive.
3) provide a TIME-LIMITED subsidy for energy storage. It should be in terms of max amount and must be able to hook up to the utility, company, or resident and provide the power. In particular, Utilities should be encouraged to move from 1 big grid, into small grids in which a storage is sitting there between the local grid and the big power grid.
With this approach, it will help utilities convert to storage, and lower their costs of energy production. In addition, it will stop new buildings from adding draw to the grid. Basically, it will help lower the real energy costs for all.