Power Supply Calculator
You need to be aware of a few issues if you are building your own computer system or checking out pre-configured systems. Two of the most important issues are hardware compatibility and the selection of a power supply unit that can power the computer's hardware. It is always recommended to check these out even if the computer has been assembled by a manufacturer. It is less likely that problems arise there but there have been cases where insufficient hardware has been build into a computer system causing all kinds of problems for the user.
Newegg's Power Supply Calculator is a online form that can be used to calculate a rough minimum power supply unit wattage. Many computer builders select a power supply unit that is over-dimensioned for the computer system that they are building. This is usually fueled by users on the Internet who give advice to pick a high-end power supply unit to be on the safe side.
The user has to enter data in the requested fields of the power supply calculator including information about the processor, video card, motherboard, computer memory, hard drives and optical drives. The calculator will then crunch some numbers and suggest a minimum wattage for the power supply unit.
It is always a good idea to pick a power supply unit that offers additional capacities to be on the safe side. This could be important when adding new computer hardware to the PC. It is therefor not advised to select a 400 Watt power supply unit of the calculator's suggested wattage turned out to be 396 Watt.
There are obviously other factors that play a role like picking an energy efficient power supply unit (check out the 80 plus power supply units article for pointers).
Newegg's Power Supply Calculator offers a quick calculation of the minimum wattage required for a specific computer system configuration. It is therefor a good starting point before selecting a power supply unit.