If there is really that much excess capacity, there is plenty of overhead to do something useful with it that actually offsets fossil sources. Like splitting water to create Hydrogen or CO2 capture. It's not a very efficient process, but it doesn't matter if you have that much energy to spare.

If it's really once in a while when it's exceptionally sunny that they do it, there's no point in doing this at all because the cost (and high depreciation) of the mining rigs makes no sense if you don't run them very often.

Either way it's a bad solution to invest energy into some made-up assets, considering how bad the climate crisis already is.

I'm not against crypto per se, but proof of work really needs to die. It's a relic from a bygone worry-free era that will never come back. It will only lead to ever more energy usage. And we will not be out of the woods for a century even if we do do everything possible.