Musk is one-smart-guy , but like his massive investment in his Reno Lithium Ion battery factory, rapid advancements in tech pose a threat of rendering some of his goals obsolete before fruition or full ROI. I'm an officer in a solar installation company & this same conversation came up the other day. Musk statements portray these solar-roof materials will last as long or longer than current roofing materials (although many asphalt shingles last 40yrs & tile last longer than that) even w/out an integrated solar component. Yet, even if that were possible, the pace of tech advancements will undoubtedly render even the best of current solar installations obsolete long before their scheduled 20-25yr lifetimes expire. On the other hand, the investment still makes sense & will have served its purpose as long as the payback doesn't drag-out & results in credible savings, especially if the on-site system produces all the independent energy needed during those 20-25yrs, as projected....to the point of eventually becoming free (for the last few years anyway).