When a complete computer system is assembled, a power supply must be chosen to provide adequate power for the CPU, motherboard and all connected devices such as RAM and hard drives. Power supply wattage needs are calculated based upon the number of components present in the complete computer system and their intended usage patterns.
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For the Intel Core 2 Quad Q6600 CPU, the power supply must be able to provide 200 watts of power just to cover the CPU's energy needs. This covers the power required for the motherboard to operate, but does not cover any additional components in the computer system. The average power supply size for Core 2 Quad systems is between 400 and 500 watts.
Each stick of memory requires roughly five additional watts. A system with four one-gigabyte sticks of memory requires an additional 20 watts of power.
The average hard drive requires between 15 and 20 watts of power per drive. Faster RPM drives require more power. Alternatives such as "green" drives require less energy to operate and a lower-level power supply can be used.
Optical drives, case fans and other accessories typically require five to 10 watts of power each. When in doubt, figure on a slightly larger number than a slower number because a computer system can function with too large of a power supply, but not one that is too small in its wattage rating.