Solar water heating systems are among the most popular solar technologies considered by those planning to switch to "greener" technologies or want to reduce their heating bills by making use of a free source. Solar hot water systems differ hugely, depending basically to different climate demands: some are specially designed for moderate and hot climates and others for colder climates. Some of these systems use flat-plate collectors, while others use evacuated-tubes collectors and batch collectors.

Flat-plate solar collectors are the most used type of solar panels today. The most common flat-plate collectors use a kind of liquid (water, glycol) as the heating fluid. They have a net of flow tubes (where the water or another heating fluid flows), a dark colour absorber plate (to absorb the sunlight heat) and a glazing surface. They also have an inlet and an outlet connection to connect the panel to the water system. Air flat-plate collectors are less common than liquid collectors. They use air as the heating medium, instead a fluid. Their absorber plates are often metal sheets or layers of screen.

Evacuated-tube solar collectors are made up of a set of parallel glass tubes (solar tubes). They are the best collectors for cloudy and freezing climates and for applications demanding high temperatures (evacuated-tube collectors provide temperatures between 75ºC and 175ºC).Evacuated-tube collectors involve literally a visible set of parallel glass tubes (solar tubes). "Evacuated" means a "vacuum" layer within the tubes, where it is also located heat pipes to absorb solar energy and to transfer it to a liquid medium. The vacuum allows very small heat losses and eliminates the impact of ambient temperatures, making evacuated-tube technology particularly suitable for colder climates.

Some solar systems use pumps (active systems) to move the heating fluid while others are based on gravity or natural convection (passive systems).

In some systems there is anti-freezing fluids (glycol, in colder climates), while others use potable water at the collector level.

Closed-loop solar heating systems do not use water in the collectors. Potable water is pumped to the storage tank but never into the solar collectors. It’s the anti-freezing fluid (often a mixture of water and glycol) that circulates in the solar collectors and through a coil pipe (in the storage tank) and the potable water is heated in the tank. The anti-freezing supports extremely low temperatures and protects the system from damages caused by severe cold weather. Glycol should be checked each year and changed regularly every few years (depending on temperatures and glycol quality).

Some systems use integrated tank (the batch solar system) and some use (the thermosyphon system) a storage tank on the top of the collector, while others use auxiliary and backup tanks and well separated storage tanks.

Batch systems do not involve pumps and do not demand a separated storage tank, or anti-freezing, heat exchanger, valves, sensors. A batch panel is typically very similar to a flat-plate collector, with a greater depth of at least 6 inches (15 cm). The water is heated directly in the collector (they are open-loop systems) and no pump is used to move the water (they are passive systems).

Their working principle is very basic: whenever the hot water needs to be stored in the batch system, the water pressure moves the hot water on the top of the batch tank (hot water rises naturally to the top of the batch tank).

Thermosyphon systems in their typical configuration do not have pumps and other controls. They are based on the natural convection principle: hot water rises and cold water sinks. In the case of thermosyphon systems: as the potable water (or a antifreeze fluid, in colder climates) enters the collector it warms and rises to the top of the collector and to the tank, where it is stored (that’s the reason why tanks are always above the collector).