Economic Evaluation of Solar Water Collectors on Building Walls in Israel

In Israel, 85% of homes have solar water heaters (SWH), according to legislation from 1980 that required the installation of SWH on the top 9 floors of new residential buildings. Building heights are steadily increasing, and more housing units are left without SWH. The solar energy legislation was repealed in 2025, leaving room on the roof for 15 kVA photovoltaic panels. For the Israeli population, SWH is not only the most economical way to heat sanitary water, but also a symbol of a standard of living and a very convenient way to supply hot water. The national standard requires the installation of solar collectors at a tilt angle of 45° facing south. All other tilt angles and collector orientations require an increase in collector size. This work is related to the tilt angle of 90° in all directions. The analysis includes calculations of the collector size, energy and water temperature that can be provided by collectors installed on the external walls of the buildings. The best location to install solar collectors is the lower wall of the balcony, separately for each apartment. The collectors are located in different directions of the building and their performance was analyzed for every 22.5 degrees, resulting in 16 cases. In the first and second options, five cases of building orientation were rejected due to low solar gain, the other 11 cases show reasonable results. The highest solar gain in winter is on the south wall, and throughout the year on the east-south wall. Option 3 includes all possible directions of the building walls, but increases the winter electricity backup for cases rejected in option 2. All cases of Option 2 and Option 3 are very profitable. The average collector size in Option 3 is 3.26 m2, a maximum of 3.58 m2 in the case of SWW+WNN and a minimum of 2.81 m2 for the south wall. The average electricity saving is 1,427 kWh/y for an apartment with 150 liter SWH. The average PV/I value in Option 3 is 5.38, a maximum of 6.91 for the south wall, and a minimum of 3.84 for the WN+NE case. The sensitivity analysis shows that even after reducing the life expectancy of the SWH from 20 years to 8 years, all cases remain very profitable with an average PV/I value of 2.91 and a minimum of 2.08.