how many watts to fill large octobox?

i am about to purchase a 60" octobox. i am using interfit 300watt strobes, until now they have been enough as i have shot mostly using a beauty dish. will these strobes still be powerful enough to fill a 60" octobox or am i going to be pushing them to the max all the time?

It is not 'watts' that fill or not fill a large reflector...it is the spread of light from the head.

After all, you light up a ceiling with a little flash aimed upward and it bounces down and lights up a room, right?!

Watts only results in the brightness of the illumination which is bounced back.

right i see your point, however i was referring to the brightness. i am trying to see if anyone from experience has had any issues with a similar setup, i usually see photogs using 1800 watt lights on larger boxes.

Beauty dish is more efficient than octa or softbox but there should be no problem filling the modifier.

you know what, i am realizing that i never specified that i shoot outdoors ie. have to fight the sun all the time. this box will probably see 10% studio 90%outdoors.

You'll have no problem at all filling a 60" modifier using a 300 Watt second strobe.

You'll also have more than enough power to be shoot at small apertures for greater depth of field, as well as dropping power to be able to shoot at reasonable larger apertures.

The Interfit 300 w/s Stellar has specs which state GN165. Used straight, not into a softbox or umbrella, that translates to f/16 at 10' distance to subject. Sunny 16 says for ISO100, 1/100 f/16. So if you increase shutter to 1/200 at f/16, ambient is underexposed by -1EV, while the flash exposure (per GN) is f/16 at 10'. So you can thus 'overpower the sun' out to 10', no more.

But, fire the flash into softbox or bounce against an umbrella, and you will lose 1EV (or more) in power! So assume you can 'overpower the sun' only out to 7' light-to-subject distance (or less)

Wilt wow thanks for doing that math for me. I'm pulling the trigger today