The accepted term in the industry for the amount of energy stored in the main discharge capacitors in an electronic flash power supply is watt-seconds (w-s), also referred to as Joules.

Watt-seconds is related to the power pack light output, but many other factors affect your real output including efficiency of internal electronic components, design of the lamphead, characteristic of the flash tube and reflectors and length of the lamphead cable.

Light cannot be donoted in watt-seconds. The light output of a flash is best stated in terms of Beam Candle Power Seconds (BCPS). However , watt-seconds are necessary to communicate how powerful a given flash power supply is. This is particularly important when the power supply is independent of the lamphead, since the power supply itself cannot be identified in terms of BCPS.

All else being equal, a power supply of twice the watt-seconds rating will produce twice the light output of the smaller unit. In other words, when using the SAME lamphead and reflector, an 800 w-s power supply would produce one f-stop more light than a 400 w-s power supply.