A computer rendering of a commercial-scale fusion power plant that Commonwealth Fusion Systems plans to build in Virginia, not far from Washington, D.C., over the next decade.

Not long ago, developers of the massive server farms talked about powering them with wind and solar energy. Now they’re bypassing the grid and building their own gas-fired power plants on site.

A power-plant building boom during the past six years in Texas means the state is juiced up to handle consumer demand for the immediate future. This year and next, if not beyond, the state is ...