I lived in Prescott Valley, and several years in a row saw brown/blackouts in mid-summer. Also saw them a number of times in the Peoria & I-17 area in phx (closer to cactus)
I've seen outages lasting a couple hours from components like transformers breaking, but that's a different issue from the grid itself not producing enough power to meet demand. Can't speak for Prescott Valley though.
Interestingly, the only article on a Phoenix area blackout due to high load I was able to source happened in the winter... probably caught SRP totally off guard:
