Chris, I don't know how big your greenhouse is, or how far from the house, but for my little 9X5 orchid shack, I have a 100 ft. contractor grade extension cord with a 4 way recepticle. I have an oscillating fan and a small programmable space heater plugged in to that. The fan runs 24/7, and the heater is set to come on at 50F, and turn off at 55F.
It doesn't use much juice, and so far it's keeping everything ok. Just something to consider until you can get what you need.
