I tested going back to kernel 4.19 which increased power use even more. While testing reverting to linux-image-5.10.0-7-amd64 and linux-image-5.10.0-8-amd64, I noticed that there was a message on the screen which should have been blanked (grub command line has consoleblank=5). What happens is that the screen is blanked for 5 seconds and then shown again.
# cat /sys/module/kernel/parameters/consoleblank 5 /etc/default/grub contains: GRUB_CMDLINE_LINUX_DEFAULT="quiet consoleblank=5" The message on screen is: dev/sda2: recovering journal dev/sda2: clean 4536/1517 584 files, 1757736/60668160 blocks [ 2.278775] i915 0000:00:02.0: firmware: failed to load i915/kbl_dmc_ver1_04.bin (-2) [ 2.278780] firmware_class: See https://wiki.debian.org/Firmware for information about missing firmware There is no login prompt or cursor on screen. The system does not respond to key presses. It is possible to log in remotely using ssh. It seems consoleblank is overridden by the "recovery journal". This is not a good situation, the screen should blank as instructed. Blanking the screen should save power consumption, if indeed the monitor and/or the integrated GPU driving it is set to low power. If there is something displayed on screen, it will not save power. This forces the monitor to blank: # echo 1 > /sys/devices/pci0000:00/0000:00:02.0/graphics/fb0/blank The effect is the same as pulling the VGA plug: power consumption drops with about 9%. Obviously, this not near 25%. The monitor is re-enabled by: echo > /sys/devices/pci0000:00/0000:00:02.0/graphics/fb0/blank The monitor turns on again, displays a login prompt and blanks after 5 seconds as expected. Power saved remains 9%. The monitor does not wake by keyboard as it does in Debian 10.
