Watt-seconds

Watt-seconds is the usual measurement for the power output of professional flash systems. 1 watt-second is equivalent to the power of 1 watt for a period of 1 second. It’s used because it’s a measure of raw power output independent of any lighting modifiers, angle of coverage or reflective surfaces

It’s only smaller, consumer-orientated flashguns that quote guide numbers. These give an indication of a flashgun’s range at a given aperture and zoom setting, but the test conditions are often not well specified.