580exII WS outsput

[ST77]

what is the watt sec output of a 580 ex II?

[D_CeLiRaToR]

speedlights aren't rated in watt secs they are rated by the guide number ie. 58m @ iso100

[Curtis N]

Based on my testing, I estimate the 580EX to be equivalent to about 40 wattseconds.

That's an estimate.

And remember, a wattsecond (same thing as a joule) is a unit of energy. How much light you get out of it depends on the efficiency of the system.

Hotshoe flash units and studio strobes really are different tools for different purposes, so comparing their output is difficult at best.