How big is your greenhouse? I use the force fan milkhouse electric heaters with a portable thermostat and have a box fan up high to circulate the air and heat. My GH is 8 x 16 and one heater does the job though I'll put two in there when the temps will get into the 20's outside for more than 2 or 3 hours. They system adds generally about 50 dollars to my electric bill every month. The picture shows 2 heaters, one high and one low.
Here is a link to the portable thermostats I use.
http://www.kkontrols.com/
