The US could return the favor, but the US is also way more dependent on the grid than China is, especially outside the cities. In some places, they would barely notice it. Ditto with the internet. The American public will cave long before the Chinese will.
The more likely scenario may be a battle over the South China Sea. Limited, involving proxy forces mostly. And the US will come out of it looking weak and undependable, because there is really no appetite among Americans for war anymore, despite Imperial Commander Enlytend's fervor.
But, there is no telling what form war takes once it starts. Which is why it's not a good idea in the first place.