This is a great question- I'm on the horn with Airbnb right now because my guests are requesting a full refund. Airbnb told me that I have an hour to resolve "the situation". I asked what situation that was, since I offered half off. They said "since there is no power, would you be able to provide an alternative source of power, since it is a very essential part of the listing as a lot of your amenities would depend on having power in the listing. If there would be no alternative source of power to be provided, then we would have to cancel the reservation".
I think that's kind of crappy given this happened last minute and I offered half off and my heat, running water are all working.