I think that if electricity goes off globally, there are plenty of other problems, bigger than e-payments temporarily not working.
And the only way to really stop this is to do it globally, which I find extremely difficult, especially since the world is using plenty of ways of producing, storing, transporting electricity.
No. I think that we already have the key. It's the diversity.
Even a big attack will not stop all the networks, all the batteries and so on. Temporarily halt some, yes, but not all and not for long.