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“Cities are setting these goals and striving to go from a very small percentage of renewables to 100 percent on an extremely ambitious timeline,” said Lacey Shaver, city renewable energy manager at the World Resources Institute, via email. “It’s an exciting time for city energy work.”But are 100 percent renewable cities actually … 100 percent renewable? The reality is a bit complicated—and it shows the challenges of true, “deep” decarbonization of electricity in the United States.First, shifting to clean electricity doesn’t mean that a city zeroes out its carbon footprint—residents could still be driving gas-guzzling cars or heating their homes with natural gas. Even most claims of running on “clean” electricity come with caveats: What cities actually mean is that they purchase enough electricity from wind, solar, or other clean sources to balance out the power that they use over the course of the year. For places filled with renewables, like Vermont, that’s not such a big deal. But in other areas, a city might not be using all renewable electricity in real-time. Even when the sun isn’t shining and the wind isn’t blowing, electrons still need to be flowing through the grid to keep the lights on. And at the moment, a lot of that more consistent energy comes from non-renewable sources, mainly natural gas and coal.
“There’s really no city that operates as an island in electricity,” said Joshua Rhodes, a research associate at the University of Texas at Austin. “You’re going to be connected to a larger grid.” There’s no such thing as “fossil fuel electrons” and “renewable electrons”—all power mixes together once it reaches the grid. That means even a 100 percent renewable town might, from time to time, be sourcing its electricity from fossil fuels. Because of this, Rhodes says that goals to run purely on renewables are more like accounting mechanisms than a pure description of a city’s energy sources.