PSU Query

Hi. Just wondering about PSU's and how they work. When purchasing a PSU should you first calculate the amount of power the system currently uses as a total and then you should match the power rating appropriately with the PSU? Would it hurt to put a bigger rating PSU in the system e.g 550W compared to 330W? Thanks

 

How do they work? A PSU is a transformer (changes voltage from 120V or 220V to 12V, 5V, and 3.3V) and a rectifier (changes AC to DC). PSUs are rated for a total wattage, but most often they are not capable of pushing their entire wattage down one group of wires (called rails). There's usually a rail for the P1 motherboard connector that accounts for about half the full rating to about 200W, and then the molex connectors for HDD get divided up the remainder.

In theory, you're supposed to calculate the maximum wattage each component uses, add up all the wattages, and then add rougly 20% of that sum to give yourself a nice conservative safety margin. Each component has a voltage and an amperage. Volts times amps is Watts. In practice, it's very difficult to get the information you need to do this calculation. You tend to either find the voltage and no amperage, or the amperage but it won't be broken down by voltage. I tend to use this web widget to get a rough idea (but it's starting to get out of date).

Using a PSU with a higher wattage rating is never a bad thing. It's only a problem if you use one that is too low. The thing is that higher wattage PSU's are more expensive, and a 600W PSU powering a system that only pulls 150W is a waste of money.