With VDPAU, the GPU is allowed to reduce clocks below P2 CUDA and P0 states. This is not the case with NVDEC, it forces the card into P2 CUDA state which totally prevents any downclocking below high baseclock (e.g. ~1.5GHz for a GTX 1070).
Thus power consumption with NVDEC is much higher than with VDPAU.
-> Please implement power savings when only NVDEC is used and not any other CUDA activity is going on!