My Brother in Law had a 20 foot by Ten foot greenhouse and used tube heaters in winter on thermostats. As the price of electricity rose he had to lower the thermostat and wrap the house with thick bubble wrap, in the end he switched off the Tubes and used two fan heater on frost guard. As it was his part time business he had to have at least frost guard.
If you are using the green house for just a few over winter plants the best way is to make a tent of bubble wrap away from the door and put a frost guard fan heater in which is what I normally do, a heated sand bed with bubble wrap curtains takes care of early seeds, shelves above that take care of young plants, heat rises.
Heating a greenhouse by any means is costly, Paraffin and gas cause condensation, electricity is expensive hence my fan heater with frost guard, it works out cheapest of an expensive lot. It also helps to blanket individual plants.