In this paper we use simulation to estimate the strength of back-up needed in Ahmedabad region to make the box cooker usable throughout the year. A lumped parameter model is made of double glazed cooker marketed in Gujarat. Its performance is simulated using climatic data of Ahmedabad during August, when insolation is the lowest. Simulations are done on an empty cooker, placed on horizontal surface at 8 A.M. Plate and cover temperatures are tracked till 4 P.M. at interval of less than a minute. Unassisted cooker will fail to be of use in August, because plate temperature rises very slowly and does not reach 120 degree centigrade, considered necessary. A back-up of 50 W is found necessary to make the cooker perform satisfactorily in August, and hence in other months. This will keep the cooker essentially a slow cooking solar device as it is.