I don't see why paying for the power would suddenly become an issue. If California can afford it now, they'll be able to afford it later, even if it's paid in a different currency--it's still the same amount of wealth.
There still has to be two parties to a purchase. You can have all the currency with which to pay but if the seller does not want to sell, it doesn't really matter how much you have. The very big problem is that, regionally speaking, California may wind up a willing buyer with no willing sellers and no way to compel anyone to sell to it.
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.