The optimal temperature in a greenhouse is about 80 to 85 degrees Fahrenheit, so the first and most important thing to know is how to maintain an equal temperature inside the greenhouse day and night, in both winter and summer. You need 1 square foot of solar panel for every 10 degrees you want to heat the greenhouse. So, if you live in a climate where the temperature inside the greenhouse needs to be maintained at 80 degrees Fahrenheit, then you would need 8 square feet of solar panels.