UEFI has nothing to do with power management. ACPI manages power. UEFI indicates modern firmware than BIOS and newer platform, thus improved power efficiency. This might be a correlation.
The PM subsystem is different from UEFI. Linux includes a whole range of power management functionalities, though unrelated to UEFI, like cpufreq, intel_pstate, pcie_aspm, laptop_mode, pci_pm, usb autosuspend. You realize these options are related to specific devices instead of the platform firmware (UEFI).
You achieve the same speed with MBR formatted disk. Faster boot speed is because UEFI is capable of more computation and doing more optimized device initialization.
If I want to disable my GUIs completely to save battery life, how would I proceed?
You can just disable your Display Manager (be it kde, lightdm, gdm, etc.) to run at boot, just run depending of your system:
sudo update-rc.d gdm remove
sudo update-rc.d kdm remove
sudo update-rc.d lightdm remove
With this you don't have to stop the desktop manager after booting. But if you don't want this, sudo /etc/init.d/<display_manager_name> stop will close the desktop manager after booting:
sudo service gdm stop
sudo service kdm stop
sudo service lightdm stop
This will work in any system that uses upstart, like Debian and derivatives.
In case your system uses systemd, you can use systemctl:
## All the next lines are executed as root
systemctl disable gdm ## or gdm3
systemctl disable kdm
systemctl disable lightdm
systemctl disable <name_of_the_service>
tell me battery life from the command line
For seeing your battery status you could either, install and run screen/byobu (which is a screen manager, and will show the percentage of the battery + other statistics) or running acpi -b.
close desktop environments
Check my previous response.
I know that I can hit Ctrl + Alt + F3 and be on the command line, but the desktop manager would be in the background still (I suppose).
Ctrl + Alt + F1-6 just switch you to one of the tty. All the programs you are running will continue executing on background.
Best Answer
A modern computer contains hundreds of parts that can be turned on and off or clocked faster or slower independently. The granularity is smaller than visible chips, smaller even than cores. A large part of power saving consists on turning parts on and off at the best time. Each part should be turned off when not in use, but only if it's going to remain unused for a long enough time to be worth it (when the part restarts, it needs some time and hence some power to reboot and restore its old state).
Thus good power management requires well-written drivers. Hardware manufacturers are very cagey about giving out details about their hardware to people who write drivers. Even people who write closed-source drivers under a non-disclosure agreement often lack detailed documentation — and people who are writing open-source drivers for an operating system that the manufacturer doesn't care much about have it a lot worse.
So on PCs, Linux starts at a disadvantage, not for technical reasons, but for social reasons.
Linux is used in many embedded devices and higher-end devices that run on batteries. The kernel matters a lot for battery life; most smartphones run Android, which is based on a Linux kernel.
To save battery, make sure not to run a “screen saver” (go for a plain black screen), and stay away from 3D effects (which stress the GPU). Don't keep web pages with CPU-intensive animations and other effects (Chrome has a nice view of per-tab CPU consumption). Enable CPU frequency scaling while not connected to the mains; it makes your computer slower by slowing down the main CPU, which saves power. Run Powertop to see where your power is going given the way you use your computer.