How to calcualte wattage of smps

There is no software to determine that. The only sure way would be to open up the thing, then search on the net for the load capacity of the components inside your psu.

Or, if you're lucky there would be a serial or model number that you could search on the net if that is indeed the power supply that you bought.

how??

The serial or model number may be inside the psu, though I couldn't guarantee that there would be one.

Did you buy this at a store? If so, try to get a replacement. If not, then I'm pretty much out of ideas.

First, I don't recall ever hearing the acronym "smps". Apparently you're talking about a power supply. If so, IMHO you are wasting your time trying to determine the wattage by looking at the components. Even if you had an accurate schematic it would take hours of calculations & research of the components to reverse engineer. You're asking for trouble if you use something you know this little about, especially since QUALITY PSUs are relatively cheap.

No, it really doesn't take "a lot of hours". Maybe around 1 hour or so. It's also easier than you think, it just involves a screwdriver and a lot of googling.

An example of a psu inspection:

http://www.hardwaresecrets.com/article/534/3