A crystal set can be very loud in a city, near an AM station. Must be milliwatts with a longwire antenna. Some clever circuit might harvest several stations.
A good LED is visible in room light at 1 uA, just over a uW. And barely visible in the dark at 1 nA.
Or takes out the exchange, which controls a large region. Depending on the population, 50 to 500 Km radius, I think.
That's a bad design, that the battery of such a thing can not be replaced "easily".
An AM or LW transmitter can be a megawat.
Google:
Long Wave Radio These wavelengths travel very long distances and generally use much higher power than medium wave, or FM and DAB. Typical power levels are 500 kilowatts, though some stations transmit with up to
2 megawatts (two million watts).Long Wave - World of Radio worldofradio.co.uk
A 5ESSS exchange we were setting up took only a few KW, 2 or 5, measured when we tested the batteries. Long distance only, just a dozen local lines.
Transmitting aerials are less efficient when their length is only a small fraction of the wavelength, hence the need for transmitters that produce lots of VA (with all the losses that entails).. Receiving aerials are similarly affected, but nowadays, with cheaply-available low-noise gain, the long-wave reception limit is usually set by the noise floor of the location.
Every cell tower consumes kilowatts and you need lots of them to reach the same number as a single AM MW or LW station.
Jeroen Belleman
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