Formulas:
Kcal yield for year = (Calories per day per square meter) multiplied by (number of days in growing season).





0 Equator 




30 Degrees 


60 Degrees 
3000 (average) 731 (midwinter) 
1000 ??? 

kcal

70 Degrees 
0 (midwinter) 

80 Degrees 
0 (midwinter) 

At and below a possible 120, earth
photosynthesis is very difficult. This is true in all zones where plants cannot get enough light. 
Special Cases