Kathy, I really can't tell you about the heating cost either, as we use it so seldom for heat. Just the odd chilly mornings in winter - it's very nice to turn it on and warm up the bedroom before I get out of bed.
I wonder if you'd have to run an electric heater in the greenhouse anyway, even if you did run heating ducts to the greenhouse from the main oil burner. It's possible you'd have to keep the whole house quite a lot warmer to keep the temp in the greenhouse up in the 50's. Might depend upon where the thermostat is.
If you keep the adjoining doors to the house open at night on cold nights, the house heat would augment the greenhouse heater or mini-split, as well.
I do know that our a/c cost is certainly lower when we run both the units at night in summer. The main house unit keeps the rest of the house at around 78 while our bedroom is cooler, usually around 72.
I'll take some pictures of our outdoor unit tomorrow. It is made by LG and has worked fine for 5 years, but we looked at one for our big shop and DH says if he buys another it will be a Mitsubishi. They are rated better. The outside unit is about 1/4 the size of a regular a/c compressor.
Here's the inside unit on the wall of our bedroom = 11in. tall, 36in. wide, and sticks out of the wall about 7in. The best part is there is no ducting at all and the installation takes one 2in. hole in the wall, plus a concrete pad for the outside unit. The front of this is not black, it is dark grey smoked glass.
