11-09-2018 06:53 PM - edited 11-09-2018 06:56 PM
This isn't a question that gets an easy answer due to some complexity in how we define the output of the unit.
A speedlite "flash" is typically rated via "Guide Number". Studio strobes are typically rated in something called "watt-seconds".
A "guide number" is a fairly simple idea... it describes the DISTANCE (which could be measured in either feet or meters ... but Canon uses meters) at which the flash can adequately illuminate a subject IF the ISO set to ISO 100 AND the f-stop is f/1.0.
The ISO isn't a problem, but the f/1.0 might seem confusing since you probably don't own any lenses that can shoot at f/1.0. That value was selected as the baseline BECAUSE it makes the math easy. All you do is divide the guide number by the f-stop.
E.g. if I am really shooting at f/4, I divide the guide number by 4 ... and that's the distance that my flash (at full power) can adequately illuminate the subject. If you shoot at f/5.6 then divide the guide number by 5.6. And so on.
For example, the 600EX has a guide number of "60" meters. That works out to about 196 feet. At f/4 that would be 196 ÷ 4 ... or 49 feet.
Canon (and this is not universal so don't apply it to non-Canon flashes) using a naming convention that tells you the guide number. A Speedlite 600EX has a guide number of "60". The 430EX has a guide number of "43". The 270EX has a guide number of "27". You can see the pattern... just drop the "0" at the end of the model number and what remains is the guide number in meters.
But there is a catch... speedlights have a strobe in flash head that has a reflector. Often the reflector can be moved forward or backward to cause the beam to go wide vs. narrow (so the guide number is based on the maximum possible distance).
But keep in mind, the head isn't meant to be disassembled... so whatever reflector is in the flash is all you get.
While we're on the topic... in the example where I showed guide number of 60 meters at f/4 works out to 49 feet and that may sound like a lot... but keep in mind that flashes are often used with modifiers or "bounced" off ceilings, walls, etc. and a lot of light gets absorbed in the process. So it's nice to have the power to spare.
When you switch to studio strobes, the head doesn't have a fixed reflector. You can swap the reflector or attach any number of light modifiers to it. This means values such as "guide number" are no longer meaningful if you remove one reflector and attach another. So instead, studio flashes use "watt-seconds" as a way to indicate how much power they can provide ... and it's even more complicated than that. Because the unit with the highest watt-second rating isn't necessarily the brightest.
Mark Wallace (Adroama TV) has some videos that deal with this.