'Power saver' mode in personal computers?

Question

While working with laptops 'Battery saver' mode plays an important role. But what is the main reason behind this. Does the switching mode power supply woks in low-dissipation state or it's the operating system which save power.

Answer 1

It is the operating system that does the power saving and it is at the expense of performance usually.

A "Battery saver" mode will typically limit the backlight intensity (that constitutes a significant part of overall battery usage), decrease the screen refresh rate when idle (as low as 20Hz), shorten the interval from idle to turning off the screen, decrease the drive time from idle to spindown, possibly decrease drive rotation speed (using SATA APM commands), change the ethernet network connection speed to 10 Mbps instead of 1Gbps, use power saving modes on WiFi, suspend USB devices, and make the CPU go more agressively to deep C (sleep) states, and keep the CPU and GPU voltage and frequency at the lowest possible setting most of the time.

The OS may go back to running full throttle when it needs to do something, like starting and application, or running a game. However waking from the deeper power saving modes takes time and so it will add latency that may or may not be noticeable to the user. And in "Battery Saver" mode the OS will go as fast to deep power saving modes as it can when the machine is idle again.

The PSU's operation doesn't change. It's efficiency will typically decrease in "Battery Saver" mode, as it has optimal efficiency at about 40-80% load, whereas in "Battery Saver" mode, the PSU will run in the 10-20% load range thanks to the laptop not needing much power.