The evacuated collector is usually an vacuum tube, heat pipe and fin absorber enclosed with an insulated aluminum manifold. the water contained within the gas or electric water heater is circulated through the solar collectors in a single-tank system, or a separate tank is used to pre-heat the water before it enters the conventional water heater in a two-tank system. A two tank system is usually considered to be the best option but if properly managed, a single tank system can provide between 50 and 80% of the household needs for hot water. The water being circulated is gradually heated and the system should be sized to provide between 30-90L of hot water per person per day. A reliable automatic control to operate the pump is essential. Fortunately, pumps and electronic controls have evolved and can be expected to provide over twenty years (and counting!) of service.