Ventilation of 40 cu. ft.

The selection is between a 50 CFM, 80 CFM, and 110 CFM extractor fans. Each would get a 2" layer of carbon to block the intake at the fan, expecting a 25% loss of air-flow (37.5 CFM, 60 CFM and 82.5 CFM respectively).

The question is: which would be adequat to cool and refresh 40 cu. ft. with a 150W (internal ballast) or 250W (external ballast) light source?

I would probably say to get the 80 for intake and depending on which filter you decide to go with, get the 110 for exhaust. I was running into the same issues but I'll be using a 400w. I just decided to go with the S&P TD100 for intake and the S&P TD100X for exhaust with a can 2900 (or 5900, I can't remember) carbon filter

Intake will be passive, and the lights will be way less than your 400W. I'm leaning more towards the 50 CFM fan.

Anyone else have experience with these tiny lights?