Instant is in they eye of the Electrical Engineer who designed the system, and the Utility who approved the connection. In some cases (Hospitals and sites with similar Uptime requirements) it can truly be as close to Instant as possible, with a Closed Transition transfer of power. In most cases, the transfer is Open Transition, which will include a power interruption for a few seconds.
Working, the difference matters to me. On vacation, I could care less - a few seconds means nothing to my vacation life.
Hey, we have that all the time at home! Our power blinks all the time, and then the satellite goes off and has to re-acquire signal. Did it last night and basically missed the end of the show we were watching. We need to buy another battery backup for it, as our current one isn't doing the job!!