Power consumption measurements (and optimizations for longer use on battery)

Hi everyone, I’ve been doing some power consumption testing with the Librem 15 (not Librem 13) and thought I’d share my results so you can benefit from my findings, and perhaps add your own (would be awesome if we collectively manage to find new ways to optimize power management beyond what I found here).

My measurements were made with an external wattmeter (precision is roughly 1 watt). The phantom load (aka vampire power) from the power supply, when not connected to anything, is 3 watts (for comparison, my Thinkpad T42’s power supply eats 4w of phantom load), to which you would add:

  • +1w when suspended but charging the battery
  • +7w turned on idle charging with screen off
  • +12 to +14w turned on with full screen brightness and charging
  • +12w turned on full brightness, charged
  • +8w turned on minimum brightness, charged
  • +9w turned on minimum brightness, charged, backlit keyboard turned on at the brightest setting

My findings with Powertop tunables, tested with the BYD kernel 4.6.1 and non-BYD kernel 4.7.0:

  • VM writeback timeout: no significant effect
  • enabling SATA link PM for the 4 SATA "hosts" yields a 1 watt power saving;
  • audio codec PM: turning this on instantly saves 2 watts
  • turning off the NMI watchdog: no significant effect
  • runtime PM for the four I2C Adapters (i915 gmbus vga/dpc/dpb/dpd, affecting the Broadwell-U integraphics runtime PM): no significant effect
  • runtime PM for the various "PCI Device Intel Wildcat Point-LP": enabling these all together RAISES power consumption by two watts!
  • runtime PM for Atheros AR9462 wireless: no significant effect
  • (already turned on) autosuspend for unknown USB device (8087:8001): turning this off raises power consumption by 1 watt
  • All the rest was already turned on.

Toggling killswitches (while the device is idle and those devices are not in use): no measurable effect on the wattmeter for the webcam and microphone killswitches, but the radio killswitch (which triggers airplane mode in GNOME) saves 2 additional watts. In addition to the powertop tweaks, I can thus bring down power consumption to 7 watts at full brightness, or 4 watts at minimum brightness (those numbers exclude phantom load).

Of course, if one wants to save power when running on battery, in addition to the tweaks above, make sure you’re running as few apps as possible (particularly web browsers and audio/video apps… those will eat your battery alive) and turn down the brightness (especially with the Librem 15’s huge screen).

Warning: the tweaks mentioned above are just my observations based on power consumption. I have not tested system reliability. Make sure you do your backups, test at your own risk, etc. :slight_smile: