Solar cookers do not work as reliable substitutes for traditional biomass cooking.
That’s in part because rural inhabitants in developing countries are often small plot farmers who must get up when it’s still dark out to get things going on the farm. Breakfast, the key meal of the day if you’re a farmers, is impossible to prepare before sunrise using a solar cooker.
The working urban poor have a different problem. If a family is out all day and doesn’t return until after dark, how can they prepare dinner? Also, where can you safely leave your solar cooker with food cooking when you live in a shanty town?