Well the steamdeck charger is only 45 watts. No charger is 100% efficient so let us assume it pulls slightly more from the wall, maybe 50 watts if you were playing the most intense game ever on it and charging the battery at the same time with brightness on max, etc etc…
That is about the same as a typical gaming monitor power usage. Without even counting how much power your computer draws.
Oh and if your steamdeck battery is full and you are playing something like a 2d indie game, the charger will only be drawing about 15watts fromt he wall. I know, I have measured it.
It is a lot more efficient than a desktop with a full sized monitor.
Compare it to a laptop however and it starts looking quite similar.
I have a real attraction to OLED displays, this fixes everything I had issue with the original except still only has a single USB C port.
I will be picking on up and giving my sister my deck :)