The U.S. Energy Information Administration's (EIA) latest data projects renewable energy to generate more electricity than coal in April and May of this year. It would be the first time wind, solar, hydro and other renewable energies will have surpassed the prolific resource in monthly U.S. generation.

The trend itself, though, isn't new. According to the EIA, the U.S. now consumes 40 percent less coal than it did in 2008. Meanwhile, renewable generation has shot up 60 percent in the same time span.

Back in 2015, natural gas began to outpace coal. But it's the first time renewables are catching up thanks to lower prices for wind and solar power.

Rob Godby, University of Wyoming energy economist, said the shift is an important marker of an energy transition.

"Over time, we're going to see this trend continue where renewables first produce a little bit more than coal infrequently. But then it will become more frequent," said Godby.

There is a seasonal component to the data. Coal-fired power plants tend to produce less in the spring time because of lower demand for air conditioning or heating. Much like with natural gas, the overlap between renewables and coal will likely fluctuate for years to come.

Dennis Wamsted, an editor and analyst with the Institute for Energy Economic and Financial Analysis, said that's what happened with natural gas.

"Coal and gas… went back and forth on market share through early 2018, with coal generating more in the winters and natural gas winning the summer generation battle. The final monthly crossover point occurred in January 2018," wrote Wamsted in a report.

Forty percent of renewable energy came from hydro power in 2018. Wind was just under that. EIA projects the two to switch positions sometime next year.