flathunter, good luck with the interview my friend!

so 150W is the maximum power that the NST can deliver, it may not be delivering that amount. If you can get an AC ammeter onto the mains input then you can look at the reading with the output winding shorted, this will tell you how many watts are going in when your NST is giving its 150W out. Then when you run your T coil you could check the input amps again, it may be lower and so less than 150W are leaving the NST.  Then all I can guess is find an approximate efficiency figure off the web for a properly tuned T coil and then just use that efficiency to approximate you T coil output power.

It's going to be very hard to get a real handle on output power without some expensive HV probe.

If you do ground to water pipes then try a wet finger on your water pipes when running, if you don't get a tingle then noone else in the flats should. unless you somehow establish some massive standing waves throughout the plumbing system lol.

Make sure you dont replicate teslas mini earthquake machine, apparantely he oscillated a small mass at the resonant frequency of the building he had an apartment in, after quite some time of running the whole building was slowly swaying back and forth lol, that crazy genius lol!