You do not need sophisticated methods to shutdown the power grid. There have been a few recent stories about some jerk shooting an electrical substation, taking down power for thousands of people. The grid backbone is enormous and impossible to secure.
Just drive a truck into a few key transformers or backbone connections could probably cut power for millions.
Yep, a rifle shot will comprehensively destroy a transformer. HV transformers are very expensive, very hard to make and if you need any number of them, already have lead times in the years. In fact anything that requires trained personnel, equipment and a supply chain for spares will be out of service indefinitely if the attacks overwhelm the repair capacity. And you only have to injure one linesman or mine one plyon to multiply the difficulty of repairs enormously by requiring armed escort and area sweeping for every repaired pylon hanger.
In any kind of concerted guerilla effort against it, the grid cannot be defended. That said, the first strikes will be easy enough, but anyone who is caught disabling electrical infrastructure or workers after the first few days--once the water, medical and food supplies have gone down and the lethal gravity of the situation is clear--will probably be treated unsympathetically by the locals.
There are also now enough people with solar that it may not be catastrophic. Many of those solar + battery systems have enough electronics to make syncing to grid relatively straightforward.
I wonder how quickly you would see small neighborhood subgrids pop up.
Well, maybe in more affluent suburban areas and rural areas. But in denser cities, the population density doesn't really support that.
Moreover, it's not homes you need to keep powered for critical life functions for millions of people (a few medical exceptions notwithstanding): it's hospitals, food infrastructure (e.g. some very big ovens, sterilisers, fridges, etc), water treatment and so on. Fuel refineries take prodigious amounts of electricity, and those logistics vehicles don't run on batteries yet, and reserves will only go so far. Telecoms also, or how will you order more food, medicine and spare parts? And that's all internet-based now, so you need at least some servers running and a lot of exchanges, towers and cabinets to be working. Ham radio might work to link onesie-twosie farmstead communities, but not for millions of people.
I guess it really depends if it's local, so you can evacuate and ship in supplies like in a natural disaster, or more widespread where there's no external resource buffer.
The US already used graphite bombs to disable 85% of the Iraqi electrical system in the 90s. Drones that autonomously target anything with insulator discs on it would make it rather cheaper, though.
It's still probably a war crime to target civilian infrastructure, however, so either win or deny using the drones and blame saboteurs now that the drones are cheap enough to reasonably suggest guerilla usage is possible.
I think their ability to remain functional indefinitely in populated enemy territory without a support network and increased attack range are the broader implications here. But yeah, you probably could have them all get into position and blow all lines at once using the range and loitering ability this affords.
A swarm of these drones could disable the powerlines of an entire country in a war situation.