I dont know but A PCB Einstein @ 640 W/s would take atleast 6+ Speelites to get kinda close to the full power output. It would be the same with these if they are measures the same.Thanks for the tip, but I can't buy Einstein, it's not sold here and I can't import due to importing rules, think it's about the lithium battery, and I think thy used to be only 120V , not 240V.
Not suggesting that you do buy an Einstein, though I recently got two and they are very good. But for the sake of accuracy, the Einstein doesn't have a battery in it and is allowed to be shipped anywhere worldwide with no restrictions (not saying there are not import regulations unrelated to batteries), also they run on any voltage from 95-250VAC 50-60Hz automatically, effectively you can plug them in anywhere in the world and they just work.
But international shipping is a pain and for service or warranty they have to go back to Tennessee and the Einstein also doesn't do HSS.
As for the power output, as IMG_0001 said, the small flash GN only covers a very small area, the Godox has a much wider spread so actually puts out much more light. It is very difficult to compare flash power on specs, even when you use the same figure to do it. For instance if you compare a 1200WS Profoto to an older 2400WS Profoto in the same modifier the 1200 actually gives you one stop deeper dof because it is so much more efficient, WS refers to potential energy.
There is no accurate way of comparing flash output, there is no standard, without firing them next to each other in the same modifier. But the Godox will give you much more light output than the 600-EX-RT.
Addendum: The Godox is rated at GN 52 with a circular beam spread of 65º, the 600-EX-RT is rated at GN60 with a rectangular beam pattern of a 200mm lens, or 12º. This means the area the Godox covers is around eight times bigger. In practice I'd expect the Godox to put out a couple of stops more power than both your 600's together.
Bearing in mind that 65 degrees is around 35 mm focal length and that Canon's 600EX-RT manual states GN36m at 35mm and ISO 100 and 1/1 power. Then, given Godox GN is 52m:
52/36=1.44 times the distance for the correct exposure and given the the inverse square law,:
1.44^2 = 2.
The Godox flash is then pretty much 2 stops more powerful than the Canon. So Private was pretty right.
And I think it is also good to remember that 1 more stop than a 600RT requires two 600EX-RT,
+2 stops = 4 flashes;
+3 stop = 8 flashes and so on.
Therefore a pair of those Godox flashes would be +3 stops and be roughly equivalent to 8 Canon 600EX-RT.
And then comes battery life, overheating possibilities, recycle times and so on...
Thanks again. It starting to look like getting the Godox for sure. I know that for each stop I need twice as many speedlites, that's why I have two, and only two. After that it starts to become expensive and loooots of batteries.
If you do so, keep us informed of how you like them. I was interested in some of the Godox products for a small home studio, but I am yet to decide if I trust the brand enough to hand them my precious beans. Still, some of their products like the bare bulb speedlight and the lithium battried speedlight appear to be relatively well perceived and the prices are very attractive.
I already have a PB820 batterypack for my speedlites and they have seen some abuse, but work as new, and it turns out I can use that pack on the 360 as well, neat stuff.
Only thing is, I saw a transmitter and receiver that allowed me to adjust power remotley, however it did not support HSS. Looks like I have to have one remote in my pocket to adjust power and another to transmitt from camera to both control power and have HSS, it seems like a pita, but hey, they're cheap.