Solar water heating systems circulate fluid through the collectors and storage tank either with the assistance of a pump ("active") or naturally with no pump ("passive").

Active systems

Active systems rely on pumps to move fluid between the collectors and the storage tank. A thermostat controls the pump. The fluid flows only when the collector temperature is warmer than the water in the bottom of the storage tank.  The pump can use standard AC power or DC power from a photovoltaic panel mounted near the collector.

Pros:  Active systems can have good freeze protection, they generally capture more solar heat than passive, and have lower heat loss since the storage tank and controls can be installed in an insulated space.

Cons: Active systems are generally more expensive than passive, they may require more maintenance over the system’s life, and the pump consumes electricity.

Passive systems

Passive systems use only the natural density difference between hot and cold fluids to drive circulation. Since warm water rises and cold water sinks, no pump is needed. Passive solar water heaters place the tank above the collector, often on the roof or in the attic. Through a process called "thermosiphoning," cool fluid is pulled from the tank into the bottom of the collector, where it warms up and exits the top of the collector into the tank. The process continues until the fluid at the top of the collector and the bottom of the tank are the same temperature.

Pros: Passive systems are generally less expensive than active systems, they will continue to function during a power outage, and the storage tank can be on the roof, saving space indoors.

Cons: Passive systems are inappropriate in freezing climates because potable water pipes are exposed to the night sky, the roof structure must be able to support the added weight of the storage tank, and the aesthetics of the storage tank on the roof are less popular.