A protestor holds up a solar panel during a climate change summit held in the city of Durban, South Africa. (Schalk van Zuydam/AP)

That explains why solar tends to cluster in specific California neighborhoods. (Interestingly, the authors note, the majority of solar adopters cite financial reasons, while only 26 percent say they’re installing panels out of concern for the environment.) But does it mean that rooftop solar adoption will eventually reach critical mass and trigger a fast chain reaction? John Farrell of the New Rules Project crunches the numbers and urges caution: “[I]f solar PV was being installed only once every 100 days at the outset, the peer pressure effect will take over 15 years to reduce the time between neighborhood installs to 10 days.” In other words, he says, “solar is contagious, but it’s not yet very virulent.”