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.