Bought some studio lights (my first) they are 200 watt.

[Pieter12]

If you are doing architectural interiors, you can make multiple “pops” for a single exposure, making up for the lack of power, because you will most probably will need a small aperture for depth of field.

 

[Matroskin]

Yeah, pretty much*. That is: assuming you're talking about a system with the same overall efficiency, and most importantly: the same light distribution on the subject. E.g. comparing a focused spot (e.g. fresnel) vs. an unfocused omni light (e.g. softbox) will yield dramatically different results even for the same output power. So if you measure the illumination on a subject of a studio strobe of brand X with a gridded snoot to a strobe of brand Y with a softbox, you'll get different results even if their rated output power is exactly the same. Due to differences in electrical efficiency (bulb choice, discharge circuitry), aging (bulb, caps), even two strobes with the same rated output power, but different models, may very well show significant differences in effective on-subject illumination.

The rated flash power is really just an indication of what you might approximately expect from a strobe.

* Technically, half a stop over 200W would be 200W * SQRT(2) = ca. 283W. That's close enough to 300W to call the difference half a stop for all intents and purposes.

PS: power output of strobes we express as Ws or Watt-second, not W or Watt. Note the difference. W is continuous power without time dimension, Ws is an actual amount of energy delivered. Coincidentally, Ws is therefore the same as Joule (J), so a 200Ws strobe at full output power will dump 200 Joules of electrical power into the flash bulb.

A 200Ws strobe is (theoretically) 200 Watts over a period of one second (200 * 1 = 200), but can also be delivered as 200,000Watts during one millisecond (200,000 * 0.001 = 200), etc. The difference is notable, because if you don't account for it, you'll get very confused trying to compare continuous lights (e.g. LED panels) to strobes, and might decide that a 200W LED panel works just as well for your portrait project (for instance) as a 200Ws strobe - which will very much not be the case as you'll find the exposure times with the LED panel to be arguably far too long for a portrait! Just one small letter, but a meaningful difference.

Thanks! I asked about continuous light sources. I'm just wondering - there is a rather big price difference in 200W and 300W, but it's only one stop.. (Like prime lenses )

 

[koraks]

Thanks! I asked about continuous light sources. I'm just wondering - there is a rather big price difference in 200W and 300W, but it's only one stop.. (Like prime lenses )

Alright, gotcha! I thought we were discussing strobes, my bad!
The difference between 200W and 300W is half a stop; I think that was a typo on your end.

Are you looking at 200W / 300W LED lights or (second hand) incandescent/halogen? And if LED, is the 200W the actual electrical power, or is it a 'nominal' / 'equivalent' power? I ask because this makes a big difference. For home use, many LED bulbs today are sold with an 'incandescent equivalent' rating - e.g. a "100W" LED bulb may in fact be around 10-15W, but its power output is considered equivalent to an incandescent bulb of 100W.

For most photo/video LED lights, as far as I know the real power rating of the light source itself is generally given in ads and spec sheets, but it's a good idea to verify this. It would be disappointing to purchase a 200W LED head only to find out that its actual power consumption is less than 40W. It'll still look quite bright when turned on, but once you start photographing with it, you run out of 'oomph' pretty much instantly. Of course, if you shoot static subjects only, you can just put the camera on a tripod and expose longer.

 

[Matroskin]

I completely understand what you're talking about. Specifically I'm talking about Godox sl200ii and Godox sl300ii. Definitely we can compere 2 light sources from one manufacturer
ps I believe the power in specs means consumption power. Every company has it's own methods of measuring Illuminance, but still there will no be such a dramatic difference in the results.

 

[koraks]

ps I believe the power in specs means consumption power.

Well, it's probably something close to that, but not quite. I expect that a nominal 200W LED continuous light will be an approx. 200W COB LED that is being run at close to its maximum rating. The actual power consumption as drawn from the outlet is likely a few % higher to account for inefficiencies and overhead. The spec sheet should state the actual power consumption (I think it's a legal requirement).

Illuminance is a different issue still and depends on a host of factors.