Power Management Systems
Power management comes in a few different sizes and shapes:
- CPU power management
- Display power management
- Complete system power management
Complete system power management is governed by what are called sleep states. "S3" is a useful sleep state, as a computer can go to "sleep" by saving the current state in ram using only a small amount of energy, and can wake up quickly when it is needed again.
In my brief experience with power management systems, I've learned they don't always work too well in both linux and windows. In some cases its worth it to do the work and get it operational.
So, how do you get it working? First off you should use hardware that supports it. For sleep states, you'll need to have a motherboard that supports it, as well as the components that go inside of it, including graphics cards, pci cards, etc. You'll also need an operating system that supports it. It you're using an operating system like debian or ubuntu, you should be fine.
You put your computer to sleep in a couple of different ways, but I'll just mention that you can do it manually or by using a timer to measure inactivity. Similarly, you can wake it up manually or based upon some sort of activity, or you can have it wake up on a schedule.
"Wake up" events can include a key press, a modem ring, or network activity.
ACPI
APM
Older, less common power management system.