Use of green energy hit an all-time high last year: Renewables produced nearly a third of the electricity used around the globe, according to a new report out from the global policy network REN-21.

That’s the good news. The bad news is even more green energy could have been used, but the electrical grids built decades ago can’t handle it. It turns out adapting a 20th-century grid to 21st-century energy sources is pretty complicated.

“The grid” isn’t just the transmission lines that crisscross the country. It’s everything from the generating stations to the outlets in your house. 

It handles energy from coal, gas-powered and nuclear plants, all of which use the same basic technology to make power, said historian Julie Cohn, who wrote “The Grid: Biography of an American Technology.”

“They’re just something [that] is hot and creating steam that’s spinning a turbine,” she said.

All of those turbines generate the same “alternating” current that the grid is designed to distribute. Solar power and wind power involve “direct” current.

“And it has to be converted into alternating current for it to operate and play well with the grid,” Cohn said.

Devices called inverters do that. But they don’t solve another problem with the grid, said Santiago Grijalva, an electrical and computer engineering professor at the Georgia Institute of Technology.

“Solar and wind are less predictable,” he said.

Think of a calm, cloudy day. If a renewable power source fails, those older turbine-based generators can make up the difference. “So you have some time to ramp up other generator to respond to that disturbance,” Grijalva said.

That hasn’t been a huge problem yet because variable wind and solar energy is supplementing the stable sources. We still need better ways of storing solar and wind energy so we can use it on those calm, cloudy days. 

Then there’s the fact that sometimes the grid doesn’t go to the places where it’s cheapest and easiest to build solar energy installations, said Tyler Norris, a fellow at Duke University.

“Here in North Carolina, a lot of the prime location for large-scale solar are on these, you know, these old tobacco farms,” he said.

Norris said the biggest challenge is coordinating all the states and cities and counties and utilities to invest and expand and update the grid at the same time.