A blink. That’s roughly how long it takes a desktop to lose its work, a router to reboot, or a CPAP machine to cut out. Most backup systems can restore power in a few seconds, which sounds fast — until you notice the damage often happens in the first fraction of one. Transfer time is the gap between the grid failing and backup power arriving: the window when the house has no electricity at all. With a generator and a conventional switch, that gap is usually a few seconds. With a fast battery system, it can be effectively zero.
