May a person use water heated by a solar heater on Shabbos?

Shalom, Thank you for your question. Solar heated hot water is widespread in Israel – and is sure to become more and more available throughout the world as time goes on. It is a relatively simple system. Let me give a brief description. Instead of a tank of water being heated up with an electrical element, or a gas flame, the water in the tank is connected to a solar panel through a pipe. This panel is really just a shallow metal box with black plastic tubing running up and down inside the length of the box. To close off the panel, a sheet is glass is placed over the front of it. The water in the tank runs into the tubing inside the panel. As the sun shines through the glass onto the black tubing, the water inside the panel heats up. As hot water rises naturally, the water in the panel rises up and “pushes” its way through the tubing, eventually leaving the panel through a pipe at the top, to return through this second pipe into the tank. As the tank is already full of water, this hot water that enters the tank near top, pushes new water, from the bottom of the tank, into the panel through the first pipe. This new cold water will itself get heated up via the sun, and, after running the length of the black piping in the solar panel, return to the tank. This constant movement of water will occur as long as the sun shines in the panel, causing the water to heat up, and so rise upwards through the laws of physics. So, with no electricity, no motor, pump or power, the sunlight itself heats up the water (via the sheet of glass and black piping) constantly, providing free hot water as long as there is sunshine. Now to the question of Shabat. The Talmud rules that it is permitted to heated up and cook food in the sun on Shabbat. So, one is allowed to hold an egg out in the sun in Elat, and (if it's really hot out) eat the boiled egg after it gets cooked on Shabat by the sun. However, it is forbidden (rules the Talmud) to cook food in something that was itself heated up by the sun. So, it is forbidden to put a frying pan out in the sun in Elat, and then fry an egg on the now hot pan. The reason for the difference is that cooking in the sun itself does not come under the definition of “cooking” that is forbidden on Shabbat. Whereas, the Rabbis forbade cooking in the sun-heated frying pan, as this could lead people to cooking in a fire-heated pan, something that is very much considered as forbidden cooking. (Both cooking directly over a flame, or cooking with a something that was itself heated in the fire is considered as forbidden Shabat cooking). When we examine the solar power system it would seem, at first glance to be a case of heating up the water in the sun directly (and thus permitted). The fact that the glass panel, and the black tubing direct the sunlight to the water is considered by most Rabbis to still be “direct” sun cooking. (Although there is one strict opinion on this question – the vast majority of Rabbis understand the glass as merely focusing the sun's rays, and not getting hot itself, to heat up the water). But – and here is the major point of debate about using solar water heaters on Shabbat – an issue arising when turning on the hot water tap in one's house. At that point, the hot water in the tank starts flowing out to the tap. At the very same time, new cold water is drawn into the tank from the local water supply. This new cold water will eventually find it's way into the solar panel, and become heated up. But until it reaches the panel, it will sit in the bottom of the tank (recalling that hot water naturally rises to the top of the tank, the cold water will stay at the bottom). While it sits there, the other water in the tank – which has already been heated up by the sun – will start heating this new cold water. That is, the hot water in the tank will directly start heating up the new cold water that runs into the tank, before this new cold water can find its way into the panels to be heated up by the sun. Now we, perhaps, have a situation where the hot water in the tank is equivalent to our pan that was heated in the sun. Just as the Rabbis forbade cooking food on a pan heated in the sun, perhaps it is forbidden to use the solar hot water system because by turning on the hot tap, one (without perhaps even knowing it) puts new cold water on a “pan” of sun-heated hot water. Great Rabbis have debated this question. Perhaps one of the more famous responsa on this question was a classic response written by the late Rav Ovadya Yosef zt”l, in which he examined the issue from every side – and in the end permitted the use of the solar heated water on Shabat. His logic includes questions of intent when turning of the tap, and issues of how directly one's actions lead to the influx of cold water. Because the turning on of the hot tap is so far removed from any intent to heat up the new cold water, and because the new cold water will in any even be heated up by the sun, this is not considered as a forbidden act of direct cooking. Other Rabbis have forbidden it's use – notably Rav Shlomo Zalman Aurbach zt”l in Shmirat Shabbat KeHilchatah (2nd and 3rd editions). He was very concerned by the fact that in winter we heat the water up with electricity or gas (in the same tank and use the same tap), perhaps leading to a situation where using the solar heated water could cause one to come to use the forbidden water in winter. There are very many other arguments made on both sides of the question – too many to be quoted here. Just be assured that every possible claim for and against has be raised by some of the leading Rabbis of the last 50 years. So – after seeing that there are those that forbid and those that permit – what should you do? Firstly you should turn to your own Rabbi for his ruling. Also, local custom may play a rule in your decision. However, if you have no Rabbi, or are not part of a community, then, in my humble opinion you may conduct yourself in line with the lenient opinion and use the solar heated water. This is because the issue at hand is a Rabbinic one, and we generally rule that in cases of Rabbinic doubt one should rule leniently. I hope this is of some help to you - Blessings.