@Alec152 I'd be saying that's a hard NO from me. A power outage has nothing to do with you, and these guests are being unreasonable. in the final 45mins they should have been packing up, and perhaps they lost the chance to have a cup of coffee. They are making a mountain out of molehill here. At the most i'd offer them $5pp to go and have a coffee locally. or divide the nightly rate/24 and refund 1 hours of time. I've had this happen, the power went out briefly, tripped the water pump which wouldn't restart, and guests were without water for an hour until I could resolve it. They shrugged it off and got on with their life. No one asked for a refund.
They are using this as an excuse, and they'll probably leave a bad review, but what will they say: power went out, host wouldn't refund. pfft. that'll make them look unreasonable.
In fact if I were you I'd mention in the review this behaviour, to warn future hosts: the power went out for 45mins on morning of departure and guests requested a refund for this "inconvenience".