One design of solar hot water heater can work right through a power failure. Called 'geyser pumping', the mechanism percolates like a coffee maker, using the sun's power to drive bubbles up through the panels. Bubbles that form in the panels lift the working fluid up out of the panels, where the bubbles are released. This allows the fluid to travel down through a long loop that takes it down through the basement-mounted hot water storage tank, where it gives off its heat to the cold water entering the tank. Then the now-cold fluid continues back up to the panels, where, joined by the bubbles, it enters the panels to be heated again. The fluid in the panels is freeze-proof, and is separated from drinking water by copper heat exchangers, so the system will withstand New England winters without trouble. While a little complex to first explain, these systems are in fact simple in operation, in that they do not depend on electric pumps, motors, valves, switches or any electricity at all. This means sun can heat your water when there's no electricity, providing an additional margin of security in tomorrow's uncertain future. Moreover, this simplicity provides outstanding reliability, and eases diagnosis and repair on the rare occasion of failure. Decades ago I helped install a predecessor to this type of solar hot water system. This newer system shares with it's predecessor the ability to shut itself down once all the tank water is hot, preventing damage and extending reliability. This is also done without switches motors valves or electricity, so this system will not overheat in a power failure. A tank below the panels accepts most of the working fluid when the system's heat fills the panels with vapor, which occurs when the basement tank has been completely heated through. Then when hot water is used, cold water replaces it in the bottom of the storage tank. This cools the working fluid returning to the panels. This cold fluid cools down the panels again, drawing the rest of the fluid back up out of the overflow tank beneath the panels, and returning the system to operation. I am really impressed with the careful thought guiding this system's design, and recommend it for Jamaica Plain residents seeking trouble-free solar hot water. For more information, see Sunnovations' archived site.