Nanosolar promises solar power at lower cost than coal, Gristmill's David Roberts reports. If they can deliver -- and the commenters are quite skeptical -- it would be a major milestone. Indeed, if solar and other renewable power sources could compete with carbon-based energy sources, it would make a transition to a low-carbon economy relatively easy-- and if such power sources are cost-competitive, the transition will occur without need for government subsidies and mandates. So, the question is whether Nanosolar and the like can deliver.