[Havoc]

For those less keen on mains voltage - some wifi smart plugs can be flash with tasmota without opening them.

Tricky but is you need a fan that powers on when it gets juice. Something that requires a button press won’t work

[baybal2]

A neat trick: there is almost always enough AC on the neutral, even when the line is off to run some very low power MCU, or even do some LED blinks.

[OJFord]

Between neutral & earth? That's surely going to depend on the topology of your earthing system [0] - TN-C will be so low impedance you won't get much through your deliberate load at all, more with TN-C-S, but most luck with TN-S where neutral already has a long cable run full of impedance before it joins back up with earth, and you can add a small enough load that that path gets a non-negligible share of the current.

[0] - https://en.wikipedia.org/wiki/Earthing_system#IEC_terminolog...

[baybal2]

Even device's own ground plane some times works. Though, it's near impossible to do with isolated power supplies. Modern ULP MCUs are incredibly power efficient, and can run for a day on a power of a single LED blink.

Anything wireless is still out of the question unless real ground is there though. Without ground, a dedicated power harvesting circuit would certainly provide more power.

We experimented with harvesting, with 10cm antennas, it's just enough to do a single beep once 20-30 minutes using specialised ULP UHF radios, or RX for a few seconds once a minute.