The Shulhan Aruch (318:3) rules that cooking with a surface heated by the sun, “Toldot HaHama,” constitutes Bishul (cooking) M’drabanan. That is, it is Rabbinically prohibited to cook an egg on a cloth heated by the sun. The reason for this is that one may come to use a surface heated by fire to cook, which is a Torah prohibition.
If one would immerse an egg in water heated by the sun, it would be an Issur D’rabanan- Rabbinic prohibition of cooking. However, Hacham David, in his Halacha Berura, permits pouring water heated by the sun over a raw food. He reasons that since there is a general disagreement whether pouring even regular hot water over a raw food can cook it, one can be lenient in the case of solar heated water, which is only an Issur D’rabanan. This question is particularly relevant in Israel where they commonly use solar panels to heat the water.
While one may not cook raw food by immersing it in water heated by the sun, it is permitted to pour such water over a raw food.