It wouldn't continue uninterrupted forever.

Those power exports are paid for in currency denominated by the US dollar.

California is no longer part of the US, remember?

Each of those states would have to negotiate with the new Republic to receive something in equal worth to the power they were delivering.

And if they don't get it, they're not going to give away power for free. Their governments won't let them, their constituents won't let them, their budgets won't let them, and lastly, the FERC won't let them.