This is the set up I'm currently looking at.

Hi / Med / Low
4220 / 3250 / 2130

1.5 amps
180 watts

My greenhouse also uses a similar exhaust fan as you mention and it works well on the thermostat. Just a note on the second layer of poly- unless you are planning on using air inflating fans between the layers to add some heat retention, there will be little advantage to installing one layer of plastic on top of the other layer.

Seems to me that 180 watts isn't enough power.

@oakhill (zone 9A, Calif.) That's the plan, using a fan between the layers of poly. We're in the greenhouse today getting it all wired and plan to install the fan and louver so that I can start some greens and carrots tomorrow. I ended up going with the unit I posted above. It looks like very good quality stuff. I'm excited!

@John D Zn6a PIT Pa that's the spec copied directly from the sellers site. Hopefully its correct, but if not, we've got plenty of power to accommodate.

I meant that going by the amount of wattage (180) there doesn't seem to be enough ventilation.

Also if your fan runs on a thermostat you also need input vent/vents that also open when the fan runs.

We got everything installed and I'm in here now feeling the cool breeze blow in, which is cooling out down nicely. The intake louvers don't open all the way with the exhaust fan on the highest speed. Is this normal, or indicative of too small of an exhaust for the area? I used the sites calculator to determine which size I needed..

You're good. That's about how much mine open. All that matters is that the cool air is getting in - which it obviously is. :)

I'm just getting around to setting my hoop house up for this season, but I've had the thermometer in there for a week. It read 121 when I came home from work today. Granted it was sunny and 55 out, and it was closed up with no exhaust hooked up yet. Funny - when I first set it up 16 years ago, I was worried about heating it. I quickly learned that cooling it is the real challenge!