When you want to buy a power supply for your computer, it is very difficult to choose the power supply wattage, and which power supply to choose. If you are using a very basic configuration normally it does not matter, but when you have graphics cards, sound cards, and other things connected to your computer it is a bit difficult to calculate.
If you know the components of you computer, (If you don’t know the components you can find details using cpu-z) you can calculate the wattage required.
Open this link (it have two modes Basic or Expert, if you know all details of your computer choose expert, which will give you a more precise calculation.
Also it will suggest a SMPS for your computer too.
The basic edition will calculate based on the Processor, RAM, Video cards Optical drives and storage devices, while the expert mode help you to choose the right PSU after considering so many items such as USB devices, Fans, Cooling kit and many other things.