Help choosing greenhouse heater

Hello everyone

I'm fed up of loosing all my plants, geraniums, dahlia corms, etc, every time I try to overwinter them, so I'm finally TRYING to buy a heater for my 6x8 greenhouse.

So far all I've come up with is Parasene superwarm 4.There is no power supply so I'll be getting parrafin. It looks ok, but has anyone used these or something like it. Can you tell me what you all use in greenhouses, please.
thanks xx