45 watt solar panel from Harbor Freight Tools (http://www.harborfreight.com/solar-panel-kit-45-watt-68751.html) $190 (often on sale in their stores for $145)
12 Volt Deep Cycle Battery from auto or marine store $80-100
12 volt (.52 amp), 102 CFM “muffin fan” from Surplus Center (http://www.surpluscenter.com/item.asp?item=16-1445&catname=), the type of fan used to cool desktop computers. $4.99
DryerDock Vent Hose Quick Connect (http://www.homedepot.com/h_d1/N-5yc1v/R-202531983/h_d2/ProductDisplay?ca...) $7.28
Extra Heat Dryer Vent Diverter (http://www.homedepot.com/h_d1/N-5yc1v/R-100167911/h_d2/ProductDisplay?ca...) $6.99
8 Ft Semi-Rigid Aluminum Dryer Duct (http://www.homedepot.com/h_d1/N-5yc1v/R-202214660/h_d2/ProductDisplay?ca...) $9.99
Hook up the solar panel to the battery per instructions mounting it high enough to avoid snow drifts.
Connect the black and red muffin fan leads to the battery with alligator clips or just put them under the wing nuts. Ignore or clip the yellow lead.
With a jig or sabre saw cut out one end of the Vent Diverter to match the shape of the muffin fan and drill holes to match the holes in the muffin fan and bolt the fan to the diverter.
Connect the duct to the diverter and then to the Vent Hose Quick Connect. Cut some slots in the most distant end of the Quick Connect to allow air to escape if the plastic gets pressed against it.
Cut hole in the inner layer of plastic and insert the quick connect, twist and snap together. Adjust the diverter flap for the right amount of air. We find it can be half open for our 96 X 26 greenhouse.
Since the fan only draws .52 amps, and volts = amps X watts, it only draws .52 X 12 or 6.24 watts. Since the solar panel is 45 watts it easily to keeps the battery charged on extended cloudy days.
The solar panel can be useful for other things. The charge controller has a nice voltage display, and comes with two 12 volts CFLC lights, USB, 3 volt and 6 volt charging jacks, and cigarette lighter type sockets for inserting an inverter or other device.
If its really cloudy a lot you can add another battery in parallel (positive to positive, negative to negative).