Drainback systems use a pump to circulate fluid through a solar loop, which is stored in a drainback tank when the pump stops.  A differential controller starts the pump when sensors attached to the collector detect sunlight.  The collector must be installed above the drain back tank.  Tanks are typically sized at 10, 15, and 20 gallons.  Each collector hold 1.5 gallons.  They are made of steel or stainless steel