I have a 400 watt light in my attic, and I use a 50 cfm bathroom exhaust fan. I know most people here will say that's terribly inadequate, but I've had three successful grows, so it can't be too bad. Granted, it does get pretty hot in there, but the light is off during the day when it's really hot. I could probably get better yields if I controlled the temperature better, but I'm content with the yields I get.