Sorry – the answer is not that simple. For one thing I do not know your climate. You will have to tell me. And next: how warm do you want to keep it in side: over wintering outdoor plants requires less heat than, orchids or growing form seeds during the winter.
Secondly, it greatly depends on what you are doing with your greenhouse – for example the more plants the fuller it is the more natural heat is retained and generated. There is a big different between half full and full: can make 10-15º difference. Large raised beds are a great heat generator.
Another option is: to use 1 or 2 large open troughs – 70 gal. + (full with water) (from say Tractor Supply) – these to can add 10-15º at night. And at the same time they offer additional humidity during the cold winter months.
One of our major customers offers the following calculator: http://igctest.commercev3.com/greenhouse_btu_calculator
Generally you will need to experiment.