Rooftop Solar Water Heater

A rooftop solar water heater uses a collector panel, usually set on a flat or pitched roof, to warm water that is stored in an insulated tank for household use. Many simple systems place the tank above the collector so heated water rises into it naturally (thermosiphon), which is why the familiar rooftop combination of panel and horizontal cylinder appears on so many flat roofs around the Mediterranean and Middle East. In Israel the system is called a dud shemesh ("sun boiler").

Israel: the 1980 solar building law

Israel's experience is well documented. During a fuel shortage in the early 1950s, when water heating was banned in daytime hours, engineer Levi Yissar built a prototype and in 1953 founded NerYah, the country's first commercial solar water-heater maker. By 1967 about one household in twenty heated water by sun, but cheap oil then slowed sales. After the 1970s oil crisis, Harry Zvi Tabor developed the prototype of the heater now found on most Israeli roofs.

In 1980 the Knesset required solar water heaters on new homes, widely cited as the first solar building obligation in the world. As summarised by solarthermalworld.org, the rule applies to new buildings under 27 m (89 ft), about 8-9 floors, other than industrial and trade buildings and hospitals; it was widened in 2012 so that taller new residential buildings must serve the top seven floors. Minimum tank sizes are 60 L (16 US gal) for a one-room flat, 120 L (32 US gal) for two or three rooms and 150 L (40 US gal) for larger flats. Over 90% of Israeli homes now use solar water heating, which the energy ministry estimates saves about 2 million barrels of oil a year.

Roofing considerations

• Load: a full tank adds a concentrated load; frames must be fixed to the structure, not just the waterproofing.
• Penetrations and fixings: frames and pipes cross the roof membrane, so flashings or raised plinths are needed to keep the waterproofing intact.
• Space: on apartment blocks, many heaters compete for limited roof area, which is one reason Israel's original rule exempted tall towers.
• Maintenance access: collectors and tanks need clear access routes for servicing and for re-roofing.

Beyond Israel

Photovoltaic panels increasingly share the same roofs: in 2023 Israel began requiring rooftop PV on all new commercial buildings, citing the lack of land for ground-mounted solar parks.

Sources and gaps

Comparable mandates and adoption figures for neighbouring countries: published information is limited.