The color of building materials can also play an important role in helping passive design. In hot southern climates, for example, the roofs of buildings should be light colored to allow the sun's heat to be reflected rather than absorbed. More complex design otions. Any thermal mass option requires careful design to promote adequate distribution of the solar heat. In addition, specific components can be designed into the building's shell or interior to capture and redistribute the heat from the sun's energy.
Rooftops are important sites for electric energy resources for the future.
Electric utilities in several parts of the country are currently analyzing the cost of renting rooftops for the placement of photovoltaic cells. These cells convert sunlight directly into electricity, and the systems may be tied directly into the utility grid. When the photovoltaic system produces more electricity than the house is using, that electricity flows back into the utility's wires, and it is purchased by the utility at a competitive rate. When the house requires more electricity than the photovoltaics can provide, it is purchased from the utility in conventional fashion. Active Systems Active systems involve moving parts that circulate air through a building or move a liquid, most often water.