I'm not sure about Texas, but lots of regions end up selling or even giving away excess power overnight that's not utilized by the grid. Not all generators can be flipped on and off like a light switch. Nuclear is a prime example.

Now, if they're still running during peak periods, that's an issue and miners should pay through the nose for it. Treat them like aluminum smelters and force them to cozy up next to hydroelectric dams to siphon off electricity before it even hits the grid.