Interesting site. It added 25 to 289 and came up with 322...
Aside from that it is not just the Wattage rating. Power supply voltage outputs are each rated at a specific max current. Voltage times current gives you the max wattage rating.
The amount of current required on each output type has changed radically over the last few years. +12V in particular is rarely used anymore (except for fans) and what used to be on 12V is on 5V now, You need to check the current rating on the 5V and 3.3 V outputs because that's where most of the load shows up in current systems.
The el-cheapo power supplies usaully have lower 5 and 3.3 V current ratings.