The small pump in the solar water heating system can be powered by normal AC power or by DC power produced by a solar panel (PV) power. Many people choose a "PV powered" solar water heating system since the system will be operating when the sun is present. Also, PV powered is a great choice for homes or cabins that are off the grid and do not connect to the regular utility grid.
In a drainback solar hot water system, the collectors and plumbing are drained of fluid when the outside temperature approaches freezing. Additionally, the fluid can be drained back to the reservoir to avoid overheating. Please keep in mind that drainback systems are significantly more complex than non-drainback solar water heating systems.