The key issue is the actual power requirement of your laptop, not the maximum rating of your power adapter. Looking at
HP ENVY TouchSmart 4t-1200 Ultrabook | HP® Official Store that one came with a 65W power supply (yours is probably different). So even if you have the 120W PSU with you on the aircraft, it would still only produce the requirement of the laptop load. The features, extra memory, CPU size, screen brightness (plus everything else) is going to determine the actual power requirement. Just buying a lower rated PSU may not solve your issue.
If it was an issue, one strategy might be to work off battery power, then turn off the laptop and let it charge off the aircraft power.
As an ex electronics engineer I do understand the issues.
I use a larger than standard 90Whr battery in the Envy 17".
Which charges very quickly with a 120W supply.
The issue is not the running demand so much but if the running demand plus the battery charge rate exceeds the capacity the power socket's inverter can deliver.
Better safe than sorry, so I will buy a less than 75W unit (which will limit the max amps the PSU can draw) so not to pop the inverter's protective circuit breaker, which can only be reset by ground crew.
I also carry 2 other fully charged 90Whr batteries so maybe just run on batteries.
As stated earlier, it is good to know this information as I was unaware there was such a low limit.
Or maybe I'm weird and one of the few to tote around a big 17" HP laptop.
Need 1920 x 1080 that I can see / read, so 17" it has to be.
Will do a few power consumption tests to measure the actual peak demand when running and charging a flat battery.
Like I said good information to know.
Would not want to get on with 2 flat batteries and plan to change them during the flight.