So in my grow box design, I've got the potential to use as many as 30x 26w CFLs. The space is roughly 2.5ft x 3.5ft x 4ft high. It's a small space, given that the lights and the handbuilt ballast will eat up ~6in, and the pots will take another ~8in.

I was just wondering, if I did indeed put the whole 30 lights in there, giving me 780w, would that be overkill?

Do you have any ideas as far as where the cutoff would be regarding benefit vs cost?

Any thoughts on how strong a ventilation fan I'll need for that? How strong a carbon filter?

Thanks!

Im saying this as someone who did it and learned. don't do all 26 watters. It can be such a damn hassle to work with that many 23~26 watters. I tried it and then upgraded to 42 watters (which are available at home depot for $9) So much more pleasent to deal with and look at, also it seems to have dropped temps in my area by roughly 5*Farenhiet. 30 bulbs = 30 ballasts in your area. And for flouros they really do put out some heat. I still use 26w for veg(6500k) as well as side lighting since the light is so close. Im not saying my setup is perfect at all, but its starting to look better. If you think you can manage it though go for it.
People say you can never have too much light but 780w in that small a area is overkill and would be a challenge to cool. Id say just take off 200-300 watts. Your power company will thank you.
Hope I was of some help :/

30 26w lights is only 780 watts and 1.17 amps. Lots of people use a lot more just to light the house. But I would have to say, that is a lot for that space. I have 10 in a 3.5x3.5x4ft space and thats plenty for me! Like lowhz said, its hard to get around with too much heat!