It seems to be that the only real requirement for solar cooking is sunshine and the more the better.
There are a few disadvantage to cooking with solar energy.
•backpack versions tend to be very slow to cook (hour plus)
•can't cook at night
•must cook in the open away from shade (near mountains, under trees, in depressions, by large hikers, etc.)
•improper use or accidents can result in serious eye damage or fire - always use eye UV protection - Solar Cookers
When using a solar cooker, it is best to use a black pot with a lid. A thinner one is better than a thick one.
Dark Colored Solar Cooking Pots are an important part of the solar cooking process. It is through the absorption of the suns rays that the dark colors of your cooking pots really enhance the overall solar cooking process and its results.
...By using dark colored, thin cookware, the heat in the solar oven transfers more quickly to the food, thus enabling one to cook quite near as efficiently as with a traditional stove/oven range - Solar Cooker
It seems altitude does not stop one from using a solar cooker as long as there is sunshine.
The Solar Trekkers Cooker fits in a backpack and has been used at base camp on Mt Everest. This link has a video of Allart Ligtenberg cooking on Everest!
Here is Allart melting snow near Trashi Lapcha pass, Nepal
For cooking times in good condition this article (How to Make a Solar Cooker and Learn Basic Solar Cooking), such as pasta, meat, vegetables, etc.
You can even experiment in your backyard cooking a chicken as seen in this video: Home made solar oven cooks chicken (YouTube).
All you need is sun and no rain.