The Amish figure it out when everyone from miles in every direction starts showing up pleading to be taught in the old ways.
Out in this neck of the woods, it would be the Mennonites (technically, the non-Amish Mennonites, since the Amish are a splinter Mennonite culture, as I remember it).
In any case, when the power goes, their first indication might be when they rode into town and found that the general store was dark and the pay phone on the corner no longer worked. However, I'm guessing a distressed population of "them English" would have already showed up begging for guidance.
That would frankly be kind of cool.
Except for the inevitable hordes of armed looters thing . . .
(BTW, up here, the Mennonites actually use electricity, so they'd know it was time to go to fallback systems pretty early on.)