OK, some psus give an all out potential added amount of wattage, meaning that there are 3 primary voltages 3.3 5 and 12 volt . A crap psu may give results like on the 3.3V 100 watts 5volt 100 watts 12 volt 300 watts. Now you maythink thatll all add up to 500 watts, and some crap psus make this claim, when in reality the total maxxed out wattage using all three voltages come to only 400 watts. The problem is, that the max amount at a given instant for any of these voltages can only be applied for one particular voltage at a time, thus leaving the say 12 volt with less power than is needed when the 5 volt is maxxed out. Good psus will show a minimum amount always available, but still may show total amount, say 3.3 volt 150 max 5 volt 150 and 12 volt 400 max, but it will only provide a total of a little over 500 total watts, and then the better psu will sell it as a 500 watt psu. Hope this makes sense heheh