As sunshine strikes the collector, the water inside it is heated. If the circulating pump is regulated by a PV panel, the pump starts turning as the PV panel is activated by the same sunshine. This direct current (DC) motor pump moves water from the tank through the collector and back to the tank. As the sun's intensity changes throughout the day, the circulating pump also changes its speed accordingly. By the end of the day, the water in the tank has been circulated many times through the collector and has been heated to usable hot water temperatures. If the circulating pump is regulated by an electronic differential controller, a sensor at the outlet of the collector and a sensor at the bottom of the tank activate the circulating pump when the water in the collector is about 15-200 F warmer than the water at the bottom of the tank. The pump then circulates water from the collector and the tank. This process continues as long as the water temperature at the collector outlet is about 50 F higher than that in the bottom of the tank. If the temperature difference decreases further, the controller automatically shuts off the pump. Common appliance timers also may control system operation. The timer is set to operate during a period of the day when solar radiation is available to heat the potable water. It is important that the timers used in these systems incorporate battery back-up in the event of power failures. In order to avoid loss of energy from the tank during overcast days, the collector feed and return lines are both connected at the bottom of the storage tank. During normal operation, natural stratification allows the warmer water to rise to the top part of the tank. During periods of insufficient sunshine or high hot water demand, a backup electrical element in the storage tank heats the water. The check valve prevents heat loss when the circulating pump is off. The circulating pump consumes only a small amount of electricity around $5 to $10 worth per year, or in the case of PV?