See the story (video there, and transcript), from PBS.

KEN NOLAN: The city is always looking at the environmental impact. Greenhouse gas reduction is a major thing that we’re concerned about and we are always trying to improve on. But in looking at whether to buy renewable power, we really were focused on an economic decision at the time.

So our financial analysis at that time indicated to our– actually, to our surprise– that the cheapest long term financial investment for us with the least amount of risk was to move in this direction.

WILLIAM BRANGHAM: Nolan says that switching from fossil fuel energy to renewable energy will likely save the city about $20 million dollars over the next two decades. What’s more, consumers haven’t been hit with a big price increase: while residential customers across the US have seen small but gradual increases in their utility bills over the years, Burlington’s rates haven’t increased since 2009.