10 log (P2/P1) = 10 log 2 = 3 dB.
If P2 had 10 times the power of P1, the difference in dB would be
10 log (P2/P1) = 10 log 10 = 10 dB.
When I work with JT65A, and I will get with 5 watt a -30db report, and it is a borderline case to copy me, so I want to be heard, I must increase my power with 8 db (that means 32 watts) to get a -22 db report. That will be fine for JT65A. More is not needed. The JT65-HF software is capable to decoded my signal. Based on your reception report you can decide how much power is needed to be heard. With 30 watts and JT65A I have worked Australia and new Zealand, That is the other side of the world for me, only with a 10 meter end fed wire on 20 meter. Because of the logarithmic curve it’s silly when someone with a 100 watt rig buys a 200 watt rig. The difference between the two is only 3 db, nobody will really notice. If we want to be ‘green’ amateurs, than we have to reduce our power level to 5 watt or lower. 5 watt is a fine starting point. When your opponent station can copy you, than decrease your power with 10 db, if not increase to a maximum of 30 watts. That is what I meant with low power = a maximum of 30 watts. For JT65A is more power not really necessary. 15 watts or so is a nice average power level. For a lot of QRP-ers it’s a matter of principle, not more than 5 watts. Not be heard? Just waiting for the next opportunity.