The proprietary NVIDIA 270 Linux driver "out of the box" was going through 29.3 Watts on average while idling compared to 33.7 Watts when setting the driver to work in its "maximum performance" state. The Nouveau driver "out of the box" came in at 35.2 Watts. The open-source, reverse-engineered driver consumed 20% more power than the NVIDIA driver in its default PowerMizer state or just 4% when the clock frequencies were unchanged.
When simply idling within the GNOME desktop with Compiz enabled, the NVIDIA driver with its adaptive PowerMizer controls had simply kept the GPU running at its lowest power state (135MHz core) due to the minimal work encountered.
The GPU temperature during this idling was at 43.1C on average with the NVIDIA blob in its default state, 49.0C in its maximum performance state, and 50.8C for the Nouveau driver.