Solar Water Heater

A solar water heater is a panel or a bank of tubes, usually fixed to a roof, that absorbs sunlight as heat and passes that heat straight into water rather than making electricity first. Fitting one typically cuts water heating bills by 50 to 80 percent, and it can supply up to about 80 percent of a year's hot water even where winters are cold. It reduces a load rather than replacing a supply, so a conventional water heater stays on to carry the rest.

Two things decide whether it works: sun and frost. The collector needs an unshaded area facing the sun's usual path, and water already piped to it. Frost decides the type. Where it never freezes hard, heated water can rise through the collector on its own with no pump. In mild frost a pump pushes warm tank water back through the collector to stop it freezing. Where cold is severe, sunlight heats a separate antifreeze fluid instead, which gives up its heat to the water through an exchanger and never mixes with it. Size follows the people served: about 1.9 square metres (20 sq ft) of collector for the first two and 1.1 to 1.3 square metres (12 to 14 sq ft) for each one after that, against roughly 75 litres (20 gallons) of stored hot water per person per day.

The two failures that destroy hardware are opposites. Fluid left in an unprotected collector through a hard freeze expands and splits it, so a freeze strategy that does not suit the climate shows itself only on the coldest night. Stagnation is the reverse: when circulation stops, through a failed pump, a power cut or nobody drawing hot water in high summer, the collector keeps absorbing until the fluid degrades and turns corrosive. Antifreeze is therefore a consumable, replaced on a schedule. Shading arrives slowly, as a tree grows into the collector's view over a decade and removes the output the system was sized for.