Curse

Quaker · Feb 19, 2010, 03:57 PM // 15:57

Quote:

Originally Posted by Malician

CPU/GPU/alldrives run off the 12 volt rail, so you'd have to look at the 12v1 and 12v2 ratings of the PSU (usually printed on it) to see. Add em' up (say 15 + 15 = 30) and multiply by 12 and you get what they're REALLY claiming the PSU can supply. If it's 300 watts or more it supposedly has plenty of headroom to add more drives and such.

This is not accurate. The total wattage listed on the supply is "what they are really claiming". It is quite common to find that, if you add up all the rated VoltsxAmps (Watts), for all of the rails, it will add up to more than the rated total wattage. This is primarily because all of the rails have to come from one common input circuit (where the AC from the wall is converted) - it is the wattage capability of this input circuit that mainly determines the total output power. (Plus losses in each circuit, etc.)
Each individual rail is capable of supplying it's rated power when used alone, but when all the rails are being used, the total power used shouldn't exceed the overall rating (give or take, plus or minus

)

So, for example lets say you have a "200watt" power supply that has ratings of [email protected] amps (120watts), [email protected](120watts), and [email protected](25watts). Clearly the total "power", if you just add them up, is way above 200watts (265watts), but in actual use, the manufacturer is saying that you can't get more than 200watts when all 3 rails are working. So, you could hook a bunch of things to one 12volt rail and draw 120watts from it, but you'd then only have 80watts available for the other 12v rail plus the 5 volt rail.

In fact, you should be a bit cautious of power supplies whose rated total power is well below the total of the individual rails.