SolarCity : The remarkable reason that solar is going viral in these 10 American cities

Solar power continues to grow at extraordinary speed across America. More solar panels were installed in the U.S. in 2015 than in any year in history, and 2016 is expected to deliver twice as much. This is the year that America surpassed its millionth solar installation, providing an historic amount of clean energy.

Source: GTM Research / Solar Energy Industries Assocation (2016)

Why is solar growing so fast? A few key factors: the cost of solar panels had plummeted, the importance of solar is critical to combating climate change, and pro-solar policies have become mainstream.

But even these primary factors don't address a more practical question. In particular, how exactly do American households decide to go solar?

In many cases, the answer can be traced to something most of us do every single day: talking to friends and neighbors.

In fact, more than one in three SolarCity customers have gone solar because they were referred by a friend or neighbor - who was actively spreading the word about the benefits of rooftop solar power.

This vibrant network of solar enthusiasts is the basis of our 'Solar Ambassador' program - which has been instrumental in driving SolarCity's more than 230,000 installations nationwide. Thanks to our Ambassadors, solar power has often become 'contagious' across communities. Seeing solar panels on a neighbor's rooftop and word-of-mouth conversations can both be key motivators to go solar, as verified by a fascinating study from Yale University researchers.

Several US cities stand out as being remarkably 'contagious' for the spread of solar - places where the majority of SolarCity customers have originated via word-of-mouth referrals. We studied our residential installations through the end of 2015, and zeroed in on the localities where solar is going viral thanks to our Ambassador referral network. Here's what we found.

The Top 10 Most Contagious Solar Cities in the U.S.

Our top 10 most 'contagious' solar cities are dotted across several states, from Hawaii to New Jersey. They span distinct geographies, climate zones, and demographic profiles. But they all have in common a boom in solar panel installations thanks to word-of-mouth referrals.

Taking the top spot for solar contagiousness is Fort Collins, Colorado (69% of its solar customers were referred by a friend), closely followed by Kona, Hawaii (64%) and Gloucester Township, New Jersey (62%).

What is it that makes these cities highly 'contagious' for rooftop solar? It certainly has something to do with people's outspoken passion for solar energy there. But we can also speculate that it has something to do with sociology and community vibrancy - wherein neighbors and friends are enthusiastic about connecting, sharing information, and helping each other.

For example, Colorado's prominence on our list of contagious solar cities may not be a coincidence, but rather stems from the state's tight-knit social dynamics. Among SolarCity territories, Colorado has some of the highest rates of people who 'talk to their neighbors' (88.2%) and are 'active in their neighborhood' (10.8%), according to the US government's Corporation for National and Community Service. And Fort Collins in particular has one of the highest rates of volunteerism (38.2%) in the country among mid-size cities - implying the kind of community cohesion and social capital that is conducive to encouraging each other to go solar.

Only six states are represented in the list above, but other states have standout cities, too. As shown in the map below, there are several other communities where a large proportion of solar customers were referred by a friend. Consider Harwich, MA and Brentwood, NY - both of which can thank peer referrals for catalyzing nearly half (47%) of all solar rooftop installations.

What does solar contagiousness look like?

Solar installations have been cropping up in the US at an eye-popping rate in recent years. Residential solar's growth rate has exceeded 50% for four years running. In the maps below, you can get a sense of this rapid growth, and the dramatic role that word-of-mouth referrals can play.

Every dot appearing in the maps represents a new SolarCity rooftop, and green dotsspecifically represent rooftops that came by way of a word-of mouth referral. The predominance of green dots in Fort Collins (our #1 most solar-contagious city) and nearby Greeley (#9) shows how fruitful word-of-mouth referrals have radiated across Northern Colorado.

We'd be remiss not to point out the small town of Windsor in the middle of the map, where a whopping 73% of SolarCity rooftops have come from referrals (hence the many green dots). Coincidentally, Windsor was also recently ranked as the best place to live in Colorado - perhaps because life is better when neighbors encourage each other to go solar! (The only reason it didn't make our top-ten list was because it doesn't quite have 100 solar installations in total, which was the threshold for inclusion in the list.)

Similar to Northern Colorado, the Honolulu Area (#6 in our list) further demonstrates how word-of-mouth referrals can fuel the spread of solar throughout a community. Interestingly, in the map below, you can see how some Hawaiian neighborhoods' adoption of solar appears to be almost entirely referral-driven (nearly all green dots), like around 'Ewa Gentry and Waipi'o.

Solar Ambassadors and their word-of-mouth referrals have visibly helped push Hawaii to achieve America's highest solar adoption rate (nearly one out of five of households in the state have gone solar). On the island of Oahu, some 32% of single-family households live underneath a solar rooftop.

Five thousand miles east of Honolulu, the influence of Solar Ambassadors is also evident in suburban New Jersey. In Gloucester Township (#3 on our list), more than three of five SolarCity rooftops can be traced to word-of-mouth referrals. New Jersey ranks fourth place nationally for total solar power capacity, in part thanks to this strong neighbor-to-neighbor dynamic.

So, who exactly are the Solar Ambassadors who are the driving force behind all those green dots we've seen above?

Who are Solar Ambassadors?

SolarCity's community of Solar Ambassadors consists of more than 200,000 people nationwide who are passionate about promoting solar to others in their network - whether it be in the neighborhood, at the office, or on Facebook and Twitter.

Many Ambassadors are proud SolarCity customers themselves, who enthusiastically refer their friends and family to follow suit. Still other Ambassadors may not have the right roof for solar themselves, but proactively advance the solar movement by educating others about the benefits and conveying how SolarCity can help.

The fact that more than one in three SolarCity customers comes to us through a referral is worth celebrating for numerous reasons: it means there is a strong excitement about solar in American communities; it's a cost-efficient way to find new SolarCity customers (>80,000 from referrals and counting); and it's also a gift that keeps on giving. In particular, we find that customers who were referred by a friend are subsequently more likely to refer others - and to refer higher numbers of them.

Of course, all this solar contagiousness is good for the planet, but it's also pretty sweet for the pocketbooks of Solar Ambassadors and the friends they refer. Each time a referral of an Ambassador installs a SolarCity system, the Ambassador getsa $200 hat-tip, and the newly referredsolar customer gets 1 month of solar electricity on us(in addition to securingan affordable, zero-carbon power supply for years to come).

Join your neighbors. Join the solar movement.

Interested in becoming a solar ambassador yourself? Learn more here.

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Special thanks to: Fiona Li, Matt Ewing, Priscilla Lopez, Yuri Yakubov, and Camila Perez-Goddard.

Methodology note: Data is based on SolarCity installations and referral activity through December 31, 2015. Only cities with greater than 100 SolarCity residential installations were considered for inclusion in our rankings of top cities.

The designation of 'Eastern Orange County' encompasses the communities of Trabuco Canyon, Rancho Santa Margarita, and Coto de Caza; the designation of 'Greater Honolulu' encompasses the City of Honolulu as well as the nearby communities of Waipahu, Kapolei, and 'Ewa Beach.