Power consumption fluctuations need to be up in the billions of watts before power companies generally care and must do something about it. Wifi routers are limited to 1W output power, so you'd need a lot more than just the hundreds of millions of wifi routers bleating out TCP packets at the top of their lungs to take down the power grid.

Also, what the power companies really care about are changes in consumption; once they've adjusted the grid parameters to compensate for an increase in power consumption, they're happy until the consumption drops off. Using wifi or any internet traffic to destabilize the grid is just not going to work because there just isn't enough raw drain available, even if the attackers could get their timing absolutely flawlessly perfect so every wifi model popped on at once.

Hm, in my experience the difference between idle and 100% CPU usage on a modern ARM processor (e.g. Allwinner H3) is around 1 Watt. That's more or less what an LCD monitor in standby draws.

I wouldn't call that significant (as in, impacting the global energy consumption significantly) even if thousands of devices started the attack at the same time.

You can sum it and have a big number but is electricity that would be wasted anyway.