close down all battery and monitoring apps.
and then see my post here
if it drains as much as you report than while plugged in there should be a discharge shown – take a picture and show.
worst case scenario, full brightness full processor power with unlimited turbo time (my setting) and 2 external hdd. Thant can easily bring you over 20 watts.
I noticed on a new computer windows does a bunch of tasks in the background what drains a lot and makes the uncooled pc quite hot. open the task manager and resource monitor and you should find what is draining your juice.
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1981 soldered my first Sinclair computer 1K, tapedeck * 1984 build and sold IBM clones 8Mhz, 512K, 20MB HDD * 2018 messing with ultrabooks