There were about 137 squirrel-induced power outages in 2015, according to Cyber Squirrel's data. That works out to about one squirrel attack every three days or so. And, as the site notes, this is probably an undercount.
The phenomenon of squirrel attacks is a great illustration of how our perception of risk tends to be skewed away from mundane frequent things toward big, scary improbable things. Plenty of ink gets spilled on the alleged vulnerability of our power grid to cyber attacks. But as none other then the Brookings Institution has noted, "squirrels have taken down the power grid more times than the zero times that hackers have." You could say the same about Hannah Montana balloons, for that matter.
As told to The Washington Post last month, The American Public Power Association, a group representing municipal electric utilities, says that squirrels are the most frequent cause of power outages in the US, although it notes that squirrel-induced outages typically affect fewer people than outages caused by storms. "Squirrels remain the biggest wildlife nemesis because of their sheer numbers and smarts," the story concluded.
Now, at least, we have a way to track that threat.