As you can see from my comment history I'm pretty bullish on solar. I think he's right about personal (rooftop) solar. Utility scale solar is about $1/watt, ins

As you can see from my comment history I'm pretty bullish on solar. I think he's right about personal (rooftop) solar. Utility scale solar is about $1/watt, installed, in the US. Residential rooftop is more than $3/watt, installed, for retrofits. Up until now most rooftop systems have been retrofitted instead of built with the house. Retrofit or not, rooftop systems can't use sun trackers like utility scale systems. Rooftops also tend to be less optimized for insolation than solar fields. So the annual-energy-per dollar gap is even wider than the installed-watts-per-dollar gap. If I had to decide how to allocate a billion dollars between utility-scale and rooftop solar systems, I'd put 100% into utility-scale. I'll be more optimistic about rooftop systems if the US can get installed costs down. Not requiring any additional land is nice. But the US has too much open space and too high costs for rooftop installation for rooftop PV to be a cost-effective decarbonization method at present.