It is well-known that one of the works forbidden on Shabbat is cooking, as we have discussed on several occasions.
The Gemara (Shabbat 38b) explains that one may cook on Shabbat using the sun, i.e. one may place a bowl of water under the sun’s rays so that it will warm up; even if, as a result of doing so, the water will reach a temperature of “when one’s hand recoils” (approximately 45 degrees Celsius or 113 degrees Fahrenheit) which is halachically defined as the temperature of cooking on Shabbat, this is nevertheless permissible. It is nonetheless forbidden to cook using something heated by the sun, for instance, placing water into an urn that is boiling as a result of sitting in the sun in order to heat the water is forbidden. Similarly, it is forbidden to place an egg into water heated by the sun in order to cook it. This prohibition is the result of a rabbinic enactment banning this form of cooking out of concern that one will come to cook using water heated by fire, which is a Torah prohibition. It is therefore forbidden to cook on Shabbat using water heated by a solar water heater, for this water is considered an object heated by the sun.
Regarding using water that was heated in a solar water heater for washing one’s hands and the like, several of the greatest halachic authorities of the previous generation discuss this issue. Indeed, this issue is an extremely lengthy and complicated one since immediately when the faucet is opened in the house, cold water then enters the tank and is boiled there. The discussion is focused on the cold water being cooked as a result of the boiling water already in the tank which is considered an object heated by the sun. There are several other issues which are addressed as well but cannot be covered in the context of the Halacha Yomit. Maran Rabbeinu Ovadia Yosef zt”l deals with this matter extensively in his Responsa Yabia Omer (Volume 4, Chapter 34; we should point out that this response is the second to longest among all of Maran zt”l’s multitude of responses. It took Maran zt”l two full weeks to write it after having returned home from the rabbinical court in Egypt at which point he secluded himself in his room until he completed this masterpiece of a response). Maran zt”l concludes that it is permissible to use water heated by a solar water heater on Shabbat and if the water is too hot and one wishes to cool off the water with a little cold water by opening both the hot and cold faucets together, there is room to allow this as well according to the letter of the law.
Nevertheless, we must point out that the above applies only to using the water for drinking, washing dishes, washing one’s hands and feet, and the like. However, regarding washing one’s entire body on Shabbat, we must discuss this point further on a different occasion.
