Solar water heating systems use solar collectors to absorb solar radiation and transfer the heat to water. Solar thermal systems are distinct from [[Solar PV]] systems: they capture heat directly rather than converting sunlight to electricity. Solar water heating is the most efficient use of solar energy, converting 50 to 70 percent of incident solar radiation into useful heat, compared to 15 to 20 percent for solar PV panels. A typical solar water heating system provides 50 to 80 percent of a household's annual hot water needs.
Passive systems use natural convection to circulate water through the collector and storage tank. Batch (integral collector-storage) systems consist of one or more dark tanks inside an insulated box with a glass cover. They are the simplest and least expensive type but freeze-prone and suitable only for mild climates. Thermosyphon systems mount the storage tank above the collector, allowing heated water to rise naturally into the tank. They are common in tropical and subtropical regions.
Active systems use pumps and controllers to circulate water or a heat-transfer fluid. Direct circulation systems pump household water through the collectors and are suitable for climates that rarely freeze. Indirect (closed-loop) systems circulate a non-freezing heat-transfer fluid through the collectors and transfer heat to the domestic water through a heat exchanger. They are used in freezing climates.
Flat-plate collectors consist of a dark absorber plate in an insulated metal frame with a glass or polycarbonate cover. They are the most common type for residential systems. Evacuated tube collectors use parallel glass tubes with a vacuum that reduces heat loss, providing higher efficiency in cold or cloudy conditions. They are more expensive than flat-plate collectors.
In cold climates, indirect systems with a propylene glycol heat-transfer fluid prevent freezing. Drainback systems automatically drain the collector fluid into a protected reservoir when the pump stops, providing freeze protection without antifreeze. Freeze-tolerant collectors use a flexible rubber absorber that expands if the water freezes.