How Do I Know how Many Watts Does My (pc) Power Supply s?

Ok, to clarify: you can pull up to 66w at 3.3v, 150w at 5v and 156w at 12v, like Oliver Ansell said, though you can't simultaneously. The fuse rating is for an internal fuse, either a dedicated glass one or a smd/through hole one that will protect the psu from complete internal shorts. The 175w and 280w ratings are for combined load. These are not the maximum specs of each rail added together since the psu might not be able to handle the maximum output power on all it's rails because of limited cooling, non optimal components...

The sticker in the corner tells you that you could theoretically draw 216w from the 12v rail but only if the supply wouldn't need some power to run itself, and most often the -5v and the -12v are taken from the 12v rail as well.

Long story short: you can draw about 150w on the 12v or the 5v rail, about 100w each if you use both.

Phew!

if it isn't a 80+ it can expend even less than the half of the watts it says so it can change your plans

Yeah man I reckon Its a 250 w PSU as it has a fuse rating of 280w max and has+12v / 18A Max

Well, I belive that the highest watt you can get is 150, on that red wire that says 5 volts 30 amps, but I wouldn't do that, just as a precaution.

Well it is rated as max 280 watts so I would go with that number