That's really your call @Manuel3353 as you know your typical clientele better than most.
Myself I would be inclined to block over somethign as significant as power outages specific to your building. Even your understanding guest is going to be this unacceptable I would think. You run the risk of having someone stay, then getting a full refund anyway for the listing not meeting basic amenity requirements. As a guest I would be REALLY put out if i went to a place where power outages were happening and I discovered that the host knew about it in advance
However if you feel the problem is only intermittent and is more than likely going to be fixed very soon, you could perhaps just block off the next few days, accept a bookings after that and if the problem continues, let the guest know giving them the option to cancel if this wish.
A bit risky as guests only have a certain number of free cancellations and are quite likely to be miffed that you allowed them to book in the first place. You would need to be sure any cancellation happened outside the 24 hour window of check in, other wise a guest can leave a review even if they didn't stay. I can imagine a guest leaving a less than glowing review for a stay unexpectedly cancelled at short notice due to a lack of power...
Just my thoughts...
