I thought I'd also mention that w/s is not a measurement of how bright the lights are. Like I said, my 300 w/s lights measure brighter than the alienbee 320 w/s units, according to their own specification sheet. That doesn't mean that alienbees isn't outputting true 320 w/s, only that the design of the mettle is more efficient at converting electrical energy into light. It's kind of like how a 30 watt incandescent bulb puts out less light than a 30 watt mercury vapor bulb, regardless that both bulbs consume 30 watts of power.
My point is that you can kind of have an idea of what class of output a light will have by it's wattage - i'd say that my mettle C300's and teh Alienbee B800 are similar side by side as far as light output, but the exact amount of light mine puts out is slightly more. At the same time, the alienbees have a faster recycle rate and shorter flash duration.
On the other hand, less reputable manufacturers might be even less efficient or - and more likely and - a little creative in how they rate wattage.
Unlike watt second, Guide Number or Lumensecond actually measures the amount of light that is being generated.