There are lots of interesting things like this right now. For instance, there is work on hot water heaters that can store excess energy by heating water hotter when electricity is cheap and then mixing with cold water to maintain a constant temp throughout the day / night.

See also "smart grid" ( https://fr.wikipedia.org/wiki/Smart_grid ) and "vehicle to grid" ( https://en.wikipedia.org/wiki/Vehicle-to-grid )

Using the grid frequency as a price signal seems to make perfect sense. A mapping table can be published of frequency to market price, and updated periodically. The prices for outer extremes of allowable system frequency should be "+Infinity" and "-Infinity" so there is never a situation that someone let's the grid frequency go outside allowable bounds for business reasons.

Then home users can use devices with very basic built in electronics to save money or even earn money on electricity bills.

It makes sense as a concept. In practice, it may lead to very unfortunate feedback effects if deployed at scale.

Use low-precision frequency references, then. At scale it'll smear out the demand spike.

Except it's negative feedback... Which increases system stability.

You still need to centralize this though.