There are ways that things can communicate over power lines, notably Ethernet over power, which, IMHO, is crap. I am sure it has its place, but I would avoid like the plague.
However, I can see one area where some simple signalling would be very cool, and ideally in a really standardised way. Indeed, the idea that this could even signal incoming on your mains feed is not at all daft.
It actually sort of exists, it is frequency - when under 50Hz (UK) the power grid is struggling to meet demand, and when over 50Hz it has extra capacity. At present, few, if any, devices can detect or use that. Just using that could be useful, but I am thinking only a simple signal of 5 possible values.
What I am talking about is some signal that can tell a device :-
- Right now power is effectively negative, please consume some power now even if it is going to waste
- Power is cheap now, if you have a choice, now is the time to use it
- Normal - use power as needed
- Power is expensive now, if you have a choice, now is the time not to use it
- Please shut down usage if at all possible right now
Not suggesting a complex set of signals by any means.
Now, if this was inherent in the mains supply, that could be useful of a lot of devices in the home. Ideally you need some signally (i.e. not frequency, really) so that you can isolate and run your own in the house. So for a house with solar and not a sane feed in rate, you can set this based on your solar/battery status in the home. But for most people not having this, the grid can set it.
I guess, it could be a standard that is frequency as the primary key and a secondary power signally for home override, maybe.
The point is that lots of kit really has a choice. At the simplest level, things like electric emersion heater, but in my case things like my hot tub. At present my hot tub heats based solely on temperature. With a very slight bias it could target that heating when we have spare solar. Just adjust target temp by say 0.5C depending on the signal on cost of power right now. Same for water heater. Same for heating or cooling a home, e.g. even aircon. Especially true for charging anything from a mobile phone, right up to a car.
It would need to be a simple standard and somehow in the chipsets for power supplies on any and every device. A world wide standard for this would be great.