@Andy48 This guest sounds like trouble. But apart from that, when something essential goes wrong, like the electric being out, even if it's something you have no control over, you should get out ahead of the game by offering the guest a reasonable discount for inconvenience. Don't let the guest gain the upper hand by demanding a discount or refund first.
If it's just a minor thing, like the electric being out for an hour or two, don't bother offering monetary compensation, just empathize and maybe do something extra, like drop off some nice pastries.
Also see if a guest needs anything, like a battery powered lamp or a flashlight if it's dark. I have a little USB battery pack that I or guests can use to charge up phones, laptops, etc, should there be no electric ( happens not that rarely where I live). Loaning a guest a cooler with a block of ice so their food doesn't go bad if the fridge isn't working is also a helpful gesture.
The electric being out all day would be considered an inconvenience that warrants offering a slight discount for that day maybe 20%, I don't know. Many guests are easygoing and wouldn't make a big deal of it, knowing it was outside your control. But there seem to be a lot of guests these days who expect hosts to throw money at them or give full refunds for any perceived inconvenience.