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The technology to generate electricity with wind and solar has existed for decades. So why isn't the electric grid already 100% powered by renewables? And what will it take to get there?

First of all, renewables have only recently become cost-competitive with fossil fuels for generating electricity. Even then, prices depend on the location, Paul Denholm of the National Renewable Energy Laboratory told CNBC.

In California and Arizona, where there is a lot of sun, solar energy is often the cheapest option, whereas in places like Maine, solar is just on the edge of being the cheapest energy source, Denholm said. In places with lots of wind like North Dakota, wind power is cost-competitive with fossil fuels, but in the Southeast, it's still a close call.

Then there's the cost of transitioning the current power generation infrastructure, which was built around burning fossil fuels.

"You've got an existing power plant, it's paid off. Now you need renewables to be cheaper than running that plant to actually retire an old plant," Denholm explained. "You need new renewables to be cheaper just in the variable costs, or the operating cost of that power plant."

There are some places where that is true, but it's not universally so.

"Primarily, it just takes a long time to turn over the capital stock of a multitrillion-dollar industry," Denholm said. "We just have a huge amount of legacy equipment out there. And it just takes awhile for that all to be turned over."

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