"Miles per watt" (<-useless)

You might sometimes see the phrase miles per watt as relates to making amateur radio contacts over huge distances using minimal transmit power (called 'QRP').

As a concept, the ratio miles per watt is basically useless. The mathematics favours tiny power levels over incredibly short distances. If you could convince -70dBm from a signal generator to leap a one foot gap to a suitable receiver (not very difficult), then you'll have achieved about 1.9 MILLION miles per watt (without even raising a sweat).

Using 3.0 watts to reach 3,000 miles (for example) is much more difficult, but this example computes to only 1,000 miles per watt.

The ratio is about as useful as "the square-root of a pickle jar"...