Last night G6ALB and I did a test on 2m to see how little power was needed to communicate between the two of us. Andrew is 3km from me. We both use V2000 triband vertical antennas. We started the test with Andrew using about 0.5mW pep from a simple DSB rig. We then switched to CW (from a decent low leakage signal generator) and G6ALB reduced power in 10dB steps. The lowest level I could still copy his signal was -55dBm which is around 1.8nW. The screenshot above, taken from Spectran, shows his signal at that level.
This sort of power level could be generated by power harvesting, even rectified RF from the HF antenna. Now there's a thought - a free power TX.