Bad news - on a 1080 Ti it’s even worse - MUCH worse in fact!
1080p h264 video:
mpv --hwdec=vdpau → 110W at the wall (this one goes to P8, so better than a GTX 980)
mpv --hwdec=nvdec → 155W at the wall! (stays in P2 forever, card’s fans start after a while)
mpv --hwdec=off → 112W
720p h265 video:
mpv --hwdec=off → 111 W (vdpau falls back to software)
mpv --hwdec=nvdec → 155W
4K h265 video scaled down to a 1080 TV:
mpv --hwdec=off → 122W (stays in P5)
mpv --hwdec=nvdec → 156W
and somewhat interestingly:
mpv -vo sdl → 118W ([vo/sdl] Using opengl, card goes to P8 but CPU load is higher)
Of course all this is measured after 45+ seconds of each playback, because this is Nvidia, card wakes up to P2 or even P0 whenever a GL window pops up and stays like that for 40 seconds, just in case. I only reported the numbers after a minute of continuous playback.
What all this means is that VDPAU and software decoding are both super efficient, while NVDEC for some reason keeps the card in high performance/power mode, hence being super inefficient.
So Nvidia, please bring back VDPAU!