Back to top Jump to featured resources
Article filed under Energy, Energy Self-Reliant States

Solar is Contagious

| Written by John Farrell | No Comments | Updated on Apr 5, 2011 The content that follows was originally published on the Institute for Local Self-Reliance website at

Update 4/6/11: Adam responds on a listserv; his comment is added below.

Adam Browning of Vote Solar writes about a recent study of the peer pressure effect of solar PV adoption.  The linked study notes that for every 1 percent increase in the number of installations in a single ZIP code, there’s a commensurate 1 percent decrease in the amount of time until the next solar installation.  As he writes, “solar is contagious!”

I’m a data lover, so I thought it would be interesting to see what this looks like over time.  If you start with a neighborhood with 25 solar installations, where it was 100 days between the 24th and 25th installation, this peer pressure effect will reduce the time between installations to just 10 days by the 250th PV project. (see chart)

Of course, this process takes a while to unfold.  In fact, if 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. 

The second line on the chart (red) looks at the change if you start with 25 solar installations but with a time between installs of just 30 days.  By the 250th PV project, the time between installs has dropped to 3 days.  And because the lag time between installations started so much lower, the 10-fold drop in lag time takes less than 5 years. 

The basic formula – written another way – seems to be that a 10-fold increase in local solar installations will result in a 10-fold drop in the time between installations. This will hold true through the second iteration, as well.  In the neighborhood with an initial 100-day lag between installations, it will take another 15 years for the lag to drop to 1 day from 10 days, reaching this level when there are 2,500 local PV projects installed.

Perhaps I can amend Adam’s statement: solar is contagious, but it’s not yet very virulent.

Update (Adam’s reply): I would note that the current strain  (solar expensivus) is not a virulent as future strain (solar cheapus).  Minnesotans are expected to have low resistance — we are talking major epidemic levels of contagion.

Note: If only the experience cost curve for solar PV worked at the neighborhood level, since it typically shows a halving of installed cost for every 10-fold increase in total installed solar capacity (worldwide)! 

Tags: / / / / /

About John Farrell

John Farrell directs the Energy Democracy initiative at the Institute for Local Self-Reliance and he develops tools that allow communities to take charge of their energy future, and pursue the maximum economic benefits of the transition to 100% renewable power. More

Contact John   |   View all articles by John Farrell