This has been debated before, would probably range between 15 and maybe 100w on an A1A emission. No one has ever found out, be interesting to know as they are good propagation indicators for me.
The antenna array used would be very interesting too. Especially on the 20MHz and 3MHz transmissions.
Currently been getting "A" Astrakhan on 20.048.1MHz nearly every afternoon for around the last two weeks, sometimes "F" Vladivostok joins it on 20.048.2MHz but is never that strong being 319 report and in/out, where Astrakhan is very steady peaking 539 at best.
Another mystery is "V" Khiva in Uzbekistan, I hear it strong on 3.227.0MHz early mornings up to 559 report but its sister on 3.657.0MHz is very much weaker around 310 to 419 report at the same time. Different antenna or different power levels?