Study: Turning Unused Signals Such as Wi-Fi into Energy for Electronics

The maximum power of license-exempt transmitters is set by the government of the country.

In the USA, there is FCC part 15 which covers license-exempt communications and part 18 which covers 'Industrial, Scientific and Medical'.

In the USA, you can radiate much more power for ISM than you can for communications. The part 18 rules forbids any "telecommunications function".

On specific frequencies, you can use much more power for things like drying rolls of paper in a factory by blasting kilowatts of gigahertz RF or medical diathermy than you can for communications such as wifi.

The FCC guidance for wireless chargers for phones is that the power transfer has to meet part 18 rules and any communication between a smartphone and a wireless charger has to meet part 15 rules.

(Technically, in the USA the legal term is 'licensed-by-rule' rather than 'license-exempt'.)

Using more powerful wifi than you need to is antisocial, you would often be reducing the performance of your neighbors wifi. If everyone does that then everyone suffers.

Sounds like we may be better off just directly powering whatever needs it.