What is the difference between Watts and Watt seconds?

Modeling lights are measured in Watts, flashes in Watt seconds. Let’s take the Move with its 30W LED modeling light and leave the LED on for 40 sec:30 Watts during 40 seconds is 1200 Watt-seconds! 40 sec of modeling light costs us 1 full power flash at 1200 Ws.Or instead of 200 flashes at full power we can get 200 times 40 s of modeling light: about 2.25 hours. (If we compare this with a Verso with its 300W modeling light:4 sec costs the energy of one full power flash. 1 minute costs 15 flashes! Of course in practice these values may differ a little bit, but this physical correlation gives us a good idea what we can expect.