Diffusion of Innovations is a widely cited theory explaining why people do or don't adopt any kind of innovation, from boiling water to eating limes on British ships to installing telephones. The concept of innovators, early adopters, and late adopters comes from this theory. More relevant to this post is that this theory posits five factors contributing to adoption, one of which is Observability: you can easily see other people gaining benefit from an innovation. The more Observable an innovation, the more likely it is to be adopted. https://en.wikipedia.org/wiki/Diffusion_of_innovations

The other is Social Proof. Seeing what other people are doing, especially those that are similar to you in some way, can help steer your behavior, often in subtle and unconscious ways. There are studies about how simple signs like "people who stayed in this hotel room re-used their towels" or "most of your neighbors are reducing their electricity usage too" can shift people's behaviors, even without people explicitly realizing it. https://en.wikipedia.org/wiki/Social_proof

My colleagues and I used these concepts in several pieces of research on what we called Social Cybersecurity (joking that the term "Social Security" was already taken). The insight we had was that cybersecurity has very low observability, making it hard for innovations to diffuse through one's social network. That is, I don't know what your cybersecurity practices are, and vice versa, making it hard for best practices to be adopted.

One intervention we did was a large-scale intervention on Facebook to improve observability, showing that simple messages like "108 of your friends use extra security settings" did increase clickthru and adoption rates of those settings. https://dl.acm.org/doi/10.1145/2660267.2660271

We also have many other studies along similar lines, e.g. many triggers for talking about and adopting security are social in nature (https://www.usenix.org/system/files/conference/soups2014/sou...), that security settings that are more social in nature are more likely to be adopted (https://dl.acm.org/doi/10.1145/2675133.2675225), and more.

What if a substantial amount of local solar contractors are doing door to door sales? Or other locality/proximity based sales (signs, driving a car with ads on it, and the like?)

I think one factor that's missing from the explanation is the extensive media and political coverage that solar panels got: There are probably very few people by now that don't know what a roof solar panel is or who don't have an opinion on them.

So my guess is that most of those neighbors who "suddenly" decided to also get a panel, were already interested or at least curious about getting one. (In the sense of "I should totally be getting one some time, but I have no time/now idea how to start/other things are more important/etc")

Maybe the early adopter was then what changed peoples' stance from a vague idea to a concrete plan.

I'm not convinced that that's it. It's more likely that the first person who got solar installed talked to their neighbors about it, and the neighbors were convinced. It's not like after you move to a neighborhood, you're really choosing anything after that point about your neighbors.

Nobody on our street has one. I wonder if I get one it will spark a trend.

I don’t think the final conclusion necessarily follows, not with this example. Solar panels are big and obvious on top of the house. It’s not the same thing as other types of values spreading through a community. The house of a healthy person isn’t any different than that of an unhealthy one.

It could be simply that the door to door solar panel salesperson was covering that 1 km area.

Also, conspicuous conservation, people like to signal how green they are: https://www.sciencedirect.com/science/article/pii/S027249442...

I'll bet early adopters of solar panels tend to be people who calculate the fuel economy of a car before making a purchase decision, people whose, damn the paperwork, ears perk up at the idea of a rebate whether from a corp or the govt, people whose enjoyment of a bath entails thinking about how cheap the hot water was.

Proximity is not just geographical.

My family has installed solar on every house we've lived while there. There is a strong economic incentive. CT has some of the most expensive avg. electricity cost in the country (a few cents cheaper than California for comparison). And Eversource, the utility company, is universally hated. Most people want to switch from their terrible service and are looking for any reason to.

Probably a lot cheaper than a $3k subsidy for everyone, as this is only 15 $3k subsidies in each zone. Also probably a lot better to do this with everyone gets a $2.5k subsidy and the first five get the higher incentives.