Because the rating is for a new supply and as the supply ages it loses capacity you want from 150% to 200% headroom on PSU capacity.
Also, it gets more complex as you reach the edge of capacity. There are about four voltage rails in most PSUs so for each rail there is a maximum plus a total for all combined.
While your 500W model might work, it usually fails later or has odd freezes, lockups and BSODs that make the PC experience an awful one.
I have Gigabyte GTX 670, and it's recommended power supply is 550 watts. I did a wattage calculation from http://www.coolermaster.com/power-supply-calculator/, and my load wattage is 491 watts. I have a 500 watt Cooler Master power supply, the cooler master rs-500-pcar-a3. I don't know if the load is too close to the power supply's limit. How much leeway do I have on the load wattage; For example 10 watts above the limit?
Should I overclock my CPU less so I have less load wattage, or should I not worry about it?


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