Soldato
Those numbers are only arbitrary nonsense, it’s only a 240VA current limiter, they all are (99% anyway, in fact I don't know of any real dual rails TBH). The way they rate them is just spin. Look at the total combined maximum wattage for the 12v.sr4470 said:
Each rail will pull to a maximum 20A (or more, some are set higher anyway). You can't get more out than you put in. If the maximum wattage is, say 360W(12v) then the PSU can supply 30A(12V).
Assume a dual rail. That could be 15A per rail. Or 20A + 10A. And you can’t just add the specified rails together either, like if it says 12v1=18A 12v2=20A, that’s not 38A.
The problem is if the components pull more than 20A on one rail, like CF 1900’s. Then if the manufacturer has been very strict, the rail OCL will trip. That’s why more rails or one rail is better than dual rails. The Silverstone/FSP IIRC has each graphics PCI-e on different rails, no way is it going to get to 20A on a single rail per GF card. Like this:
+12V1(13A) : CPU1
+12V2(18A) : CPU2+SATA
+12V3(16A) : MB+VGA1
+12V4(18A) : HDD+VGA2
And ignore the total PSU wattage, it meaningless, more hype. Some lower wattage PSU kill the bigger ones. Add up your 12v current load, then add a bit for losses and match the 12v total wattage available on the PSU you fancy.
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Which of course they don't. Take the hard drives at the sustained read/write values, not the spin up and that's 48A/580W (12V) still not quite right. For power calculations its normal to include factors for diversity (i.e not everything runs flat out all together), loss (inc derating) and efficiency. I usually work on stuff a bit bigger than a PC, but say the running load is only 80% of maximum, but the losses to heat and resistance are 5% and the PSU is down 5% from age (or its not a review sample). At a conservative estimate say you need 90% of the maximum load 43A/520w at 12v. And remember that's now a constant sustained load, not a millisecond peak value. Watch it, because some firms like to quote peak or they rate the unit in a freezer. Look for the ones rated at 50C, PSU's derate very quickly, like 10W/C or -100W for a delta of 10C and so on.
