America’s clean energy revolution is on the verge of a tipping point.

The renewable energy sector is projected to generate more electricity than coal during the month of April, according to a recent report published by the Institute for Energy Economics and Financial Analysis. That’s never happened before, according to CNN.

Coal, long the king of the power sector, has already been dethroned by natural gas, a much cleaner burning fossil fuel. Now, coal is facing intensifying pressure from wind and solar power.

“Five years ago this never would have been close to happening,” Dennis Wamstead, research analyst at IEEFA, said in an interview. “The transition that’s going on in the electric sector in the United States has been phenomenal.”

The IEEFA report cited US government statistics that project renewable energy (hydro, biomass, wind, solar and geothermal) will start to sporadically exceed coal in 2019 and 2020. In other words, this year renewables will likely start to generate more energy than coal’s 240 gigawatts of remaining capacity, at least on a month-by-month basis, according to the US Energy Information Administration.

While such a milestone would highlight the shift away from coal, some of it is also because of seasonal issues.

For instance, some coal plants shut down for maintenance during the spring when demand for electricity is low. The spring also tends to be a strong period for hydro and wind power.

Renewable power isn’t expected to surpass coal on an annual basis for at least several years.