David Keith, Gordon McKay Professor of Applied Physics at Harvard’s John A. Paulson School of Engineering and Applied Sciences and Professor of Public Policy at the Harvard Kennedy School, has changed his mind about solar power.

Over the last few years solar PV has got cheap. Cheap enough to start impacting some commodity energy markets today. Cheap enough that with continued progress, but no breakthroughs, it might alter the global outlook for energy supply within a decade.

I have long been skeptical of solar hype. In 2008 we did an expert judgment exercise suggesting only even odds of getting to module prices of 0.3 $/W in 2030. In 2011 we did some analysis showing how the power-law learning curve for modules appeared to be flattening. That analysis was done at the end of a decade that saw big increases in installed capacity, with little corresponding change in module prices. The solar market was driven by incentives, like tax credits and feed-in tariffs that drove rooftop solar seem systems which are (arguably) little more than green bling for the wealthy. I worried that deployment incentives (global total amounting to many hundreds of billions of dollars over the past decade) would simply lock in the current technologies and do little to drive the breakthroughs that were needed to get solar cheap enough to compete for commodity power.

I was wrong.

Facts have changed. Just a few years ago the cost for industrial systems was twice what it is today. A host of little innovations have driven costs down. Module prices are now around 0.5 $/W. The unsubsidized electricity cost from industrial-scale solar PV in the most favorable locations is now well below 40 $/MWhr and could very easily be below 20 $/MWhr by 2020. Compared to other new sources of supply, this would be the cheapest electricity on the planet.

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