A good source for information on passive solar building designs is the Passive Solar Energy book. Active solar thermal systems employ solar collectors that concentrate the sun's heat and usually transfer the heat to circulating water. Because the solar collectors are very good at collecting the sun's heat, they can be used not only for keeping a living space warm, but also for providing domestic hot water. Active solar systems are more complex than passive solar designs. A typical active system has one or more solar collectors mounted on a tilted roof facing south.
Water is pumped through the collector, transferring the heat from the collector to the water.
The water is then stored in a large insulated tank inside the building. It can then be used to provide heat to the building or provide hot water. Originally designed for space missions, photovoltaics now produce energy for many cost effective terrestrial applications. Solar calculators and watches have small photovoltaic cells built into them providing power for the electronics or to charge a small battery in the device. On a larger scale, many PV cells can be combined to provide enough power for a walk light, a street light, or even an entire house.