For your information, the first general-purpose electronic computer system named the ENIAC consumed about 140 thousands watt of power as it has 18 thousand vacuum tubes. Therefore due to consuming those high amount of watt power, it produced non-trivial electricity bill. After that, there was an invention of transistor, and the power usage dropped in large amount.
Now, in today's computer world, the power management is a very useful task for many reasons. OS is playing a big role in power management.
Generally a desktop personal computer has a 100 or 200 watt power supply.
Let's suppose that if 100 million personal computers each having 200 watt power supply, are Turned On at the same instance of time in the whole world, then it means that together they use 20 thousands mega watts of electricity. And this is the total output of 20 nuclear power plants of average-size.
Now if the power supply could be cut in half, then we could get rid of almost 10 average-size nuclear power plants.
The batter-powered computer systems such as laptops, notebooks, palmtops etc. are another place where power is a big issue.
Therefore power management is also essential in these types of computer systems.
The main problem in these type of battery-powered computer systems is that the batteries can't hold the enough charge to last very long a few more hours.
Therefore everyone's main aim in making those type of battery-powered computer systems that uses less energy just to make the existing batteries last longer is high.
The OS plays a major role here in saving the battery to use the same for more time.
Generally, there are the following two things to reduce the energy consumption: