flash - How many watt-seconds do regular battery-powered flashes have?

I'd like to know how do battery-powered system flashes compare to studio units in terms of light output.

I know that AlienBees B400 has 400 "effective"  Ws, and that Canon Speedlite 580EX II has guide number 42 (meters) at ISO 100 and 50 mm zoom setting.

But how do I compare these two? Is there any way to convert guide numbers to watt-seconds?

Answer

No, there is no way to convert guide numbers to watt-seconds. Watt-seconds is a measurement of how much energy is used by the flash, not how much light is put out. A significant portion of this energy is wasted as heat, infrared, ultraviolet, etc.

A 4 watt-second flash that is 100% efficient will put out the same amount of light as a 400 watt-second flash that is 1% efficient.

"Effective watt-seconds" are ill-defined as well, so are basically just as useless as watt-seconds.

By contrast, guide numbers are fairly well defined. They can be directly compared at a given beam spread (assuming manufacturers aren't stretching a bit. Ken Rockwell seems to think most flashes are over-rated by about a stop.)

However, the most accurate way to compare two flashes is through an actual scientifically defined unit like the lumen-second. From Wikipedia,

The lumen (symbol: lm) is the SI derived unit of luminous flux, a measure of the power of light perceived by the human eye.

As photographers, we're all keenly aware that time plays an important role in exposure. The longer the shutter is open, or the longer the light is on, the higher the exposure. Thus, lumen-seconds more directly translate to exposure than lumens do.

Here's a page from AlienBees' website which includes the specs on your B400 (7000 lumen-seconds) as well as a paragraph or two about how bogus "effective watt-seconds" are.

The Effective Wattseconds rating, however, is rather arbitrary and cannot be easily proven true or untrue, as it is merely used as a basis for inflated comparison of different flash systems.

I've looked around for a lumen-second rating of the 580 ex II, but can't seem to find one.

EDIT: David Hobby, master of the speed light, keeps saying 60 watt-seconds. (Note: that last link is an April Fools joke.)