There is an interesting article at Microsoft Help which talks about Power options and then goes on to tell you where all the power goes in a Windows 7 computer. So if you are curious to know how Windows 7 utilizes the power, this article may interest you!
Display consumes 43% of the total power. On a large mobile PC that has a 21-inch LCD screen, Microsoft has found that the display consumed as much as 43 percent of the total power being consumed at high resolutions. This was especially true when the mobile PC was combined with a high-end graphics card that was tasked with video editing or gaming. An external display can consume from several watts of power to several hundred watts of power for a large plasma, LCD, or projected screen when you use Windows 7 Media Center. Newer OLED displays are on the horizon and will have a significant reduction in power consumption.
Disk usage consumes around 5%. Sustained hard disk usage consumes an average of around 5 percent of total power consumption in a mobile PC. New solid-state drives are much more efficient, because there is no mechanical mechanism and there are no platters spinning. Hybrid drives that have a solid-state and disk combo are an excellent choice. Another bonus to having no moving parts is that the hard disk is more resistant to the bumps and harsher treatment that mobile PCs experience. For servers, a datacenter-grade hard disk consumes on average 10 watts of power while in the idle state. When you multiply that by 1,000 or more hard disks in a typical datacenter, you have high current loads and additional cooling needs.
More RAM requires more power, because the chips are all powered on when you start a computer regardless of whether it has data.
Chipsets consume 21%. In terms of the total power being consumed to keep a system running, chipsets can on average consume 21 percent of the power, whereas processors (depending on the number and speed of the processors) can consume 10 percent of the power.
Network adapters consume 4% of the power. A wireless NIC may require more power. Typically, enterprise-level NICs that run in the gigabytes consume even more power.
The PC bus can also consume power. However, newer motherboards let buses be powered down when they are not being tasked. Or, you can set your system to Passive Cooling. Some manufacturers expose a great number of features that can be changed in the BIOS. Not all BIOSes expose or take advantage of these features. Some BIOSes may have these features but do not let the operating system alter the settings from the BIOS.
Attached devices such as mobile phones, TV cards, MP3 players, GPS devices, and wireless NICs all take additional amounts of power. When you attach these devices, the bus to which they are connected is powered up. Additionally, these devices may start charging their own battery. If you are using an AC power source, that is fine. But if you are using battery power or are using one or more devices that may not support the Selective Suspend feature when they run on battery, the battery run time will decrease significantly.
Drivers, services, applications, and devices can tale over 20%. They add significant demands on the battery, and any combination of them can affect battery life by 20 to 30 percent or more.