What actually I was trying to find out that why there is such a big gap between the combined powers ie 504 vs 576. The second psu (corsair) delivers almost what it says on the label whereas in the first one (high power), the power drops down almost one-sixth of the total power stated on the label. That is a bit too much I guess.
You seem to be mixing up two different things : the total power rating, and the power rating for the 12 V line(s) only. A PSU doesn't just output 12 V, it also outputs 5 V and 3.3 V (and -12 V, but that's pretty much negligible). In practice, most modern components draw power mostly (or only) from a 12 V line, but the other lines still get some use (USB ports include a 5 V power line, for instance). So, a PSU rated for 600 W total and 504 W combined on the 12V lines isn't "dropping down almost one-sixth of the total power stated on the label" - it just means that the other 96 W are only available to the 3.3 and 5 V lines. In practice, you want a PSU that can output most of its total rating on the 12V line, but both PSUs seem to fit the bill well enough in that regard (500W is quite a lot already).
I know no gpu consumes 500W ( maybe a few out there, not sure) but when using those psu calculators on the net giving you a rough idea about the total consumption, would you choose your psu according to the advertised power or the combined power?
First of all, the PSU calculators you can find on-line generally give very conservative estimates (read: they overestimate everything). So keep in mind that you should generally consider them as a worst case situation (if they say you need a 500 W PSU, you really don't need more, and you're probably fine with a 450 or 400 W one...), rather than some kind of average estimate. Of course, I can't say that's true for all
on-line calculators - but it seems to be true for most of them at least.
(edit: The numbers from real measurements given by MikeC above show just how much those on-line PSU calculators can over-estimate... Though, for fanned PSUs, most of them get way noisier when you get close to the max power rating, so you may want a bigger PSU than necessary for this reason.)
Second, what they give you is the total power consumption, regardless of the lines used, so you want to look at the total power rating of the PSU. But you also have to keep in mind a few things:
- a modern PSU needs to be able to output most of its power rating on the 12V lines, because it's what most components use;
- obviously, that means that it doesn't need very high ratings on the 3.3V and 5V lines, so you normally don't have to worry too much about those (except if it's for an old PC);
- very cheap, no name PSUs often use very misleading labels (when they don't outright lie about their power ratings) - though you don't have to worry about that if you buy from a reasonably reputable brand.