The new thing in electricity is datacenters. The new new thing is ... coal?

Two companies you've probably never heard of have signed a deal that captures a seismic shift on America's biggest power grid, with implications for household bills, climate change and the artificial intelligence revolution.

Energy Capital Partners LLC, a private equity firm, is buying Lightstone Holdco LLC, a joint venture between Blackstone Inc. and Arclight Capital Partners LLC that owns four power plants in Ohio and Indiana. Three are gas-fired. The other is one of the largest coal-fired plants in the US, the General James M. Gavin facility; briefly famous in 2001 when ...