I don't think it is reasonably considered a full "walk" (paying first night etc), provided it was a normal out-of-their-control power outage, and not a preplanned maintenance one at the hotel.

It would, however, be entirely reasonable for the customer to find an alternate hotel and insist on a full refund (if prepaid, including Priceline etc) or not to be charged (if just guaranteed). If the hotel can't deliver the product, even if not their fault, they must of course not charge for it, even if not required to compensate further (because it's not the customer's fault either).