Anyone who has ever turned on a garden hose left out in the summer sun knows beyond any doubt that the sun can produce scalding hot water.
I live in an area of northern New Mexico which has between 280 and 300 sunny days per year – a statistic closely matching the island of Cyprus in the Mediterranean Sea. Here’s a paragraph from a recent (20 September 2024) article in The Guardian newspaper about solar water heating in Cyprus where over 90% of its residents have solar thermal systems:
For Demetra Asprou, a retired engineer, it’s obvious that a region blessed with more than 300 days of sunshine a year should embrace solar energy. “It reduces electricity costs, increases the efficiency with which hot water is provided and is kind to the environment,” she says. “Why would anyone use other, more traditional means to heat up water when only a few hours of sunlight, between 11am and 2pm, is enough for a 200-litre [44-gallon] tank to be filled with warm water that will last 48 hours? On days when there is no sunlight, which is rare, you always have electricity as a backup if necessary.”
However, Cyprus has a more temperate climate than northern New Mexico and so they can use outdoor water tanks and not worry about freezing temperatures. In practice, all that this means is that slightly different equipment must be used here. What really matters is the number of sunny days. Period.
In years past, when solar electric was much more costly, there was a clear benefit to installing solar thermal systems to heat water. Now, with relatively low-cost solar panels, it can be cheaper to heat water off-grid with solar electricity. The principal benefit is that it saves a lot of plumbing work needed to move heated water from outdoors to indoors through insulated pipes. On the other hand . . .
Depending on how one’s solar electric system is configured (total kW rating and inverter output power), solar electric water heating may or may not be feasible. Specifically, smaller PV systems may not have sufficient capacity to reliably heat water on a daily basis. This is the situation where a standalone, non-electric, solar thermal system can be the right choice. Properly designed, these systems are mechanically simple and very resilient. The bottom line is that no one should be burning fossil fuels simply to make abundant hot water in a climate such as ours in northern NM (or in Cyprus).