If you know the surface area and r-value of each wall, you should be able to calculate your required BTUs. Here is one on-line calculator to help (linked below)

Here is a link that might be useful: heat loss calculator

I used the calculator in the link and it told me i needed around 20000 btu to maintain a temperature of around 50 degrees (I am not sure if I entered everything in correctly). I used another calculator on the ACF greenhouse website and it told me I only needed around 10000 btu to keep my greenhouse at 50 degrees when the temperature outside is 0 degrees.

This is what I entered in at the ACF website

Area 500
Heat loss value .38 (with bubble wrap and north wall insulated)
Minimum outside temperature 0
Inside temperature 50

Which one should I trust?