Prestige City power backup: What's the real deal?

Prestige City — I'm looking at a 2BHK there. Kids have moved out, so don't need such a big place anymore. Selling our old house, want something manageable, especially with good lift access. My main concern is power backup. They say full backup, but is it really for the whole flat, or just common areas and a few points? What's the reality like for daily living there? Does it add a lot to maintenance charges too? Trying to avoid tanker dependency, honestly.