I was in my local Wal-Mart store once when all of the electricity went out ... and, apparently, the generators didn't kick in. They cleared the store and put us out until the problem was corrected.
I later found out that the MAIN reason for that was because the store's cameras and security devices were off-line and they felt they could not adequately guard against massive THEFT during that time.
It was closed for about 30 minutes. And then ... it was back to 'shopping as usual'!!
However ... twice since that time I have been in the SAME store when a thunderstorm was raging outside ... and lightening caused the store's lights to momentarily go out ... but within seconds the generator came on ... but the cash registers went offline and the checkers could not ring up purchases!!
In both instances, we were asked to "wait patiently" as it would only be a "few minutes" (more like 20+!!) before they could resume business!!