I have to say hands down the best investment I've made for my greenhouse has been the pellet stove I recently added. After my first year of outrageous power bills with my 2 220 electric heaters this pellet stove so far has been way more economical. Best part is the one I got from tractor supply automatically turns on and off and so far doesn't use much pellets per day to keep the greenhouse from dropping below 50f here in pnw zone 8. Can't recommend it enough after 900 dollar monthly power bills last winter.
Thanks to whoever it was last year that recommended going this route.