For the spy with a yen to be green, the house has low utility bills courtesy of a passive solar design, low-E windows and solar-generated water heater.

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The design allows for passive solar heating in the winter, has a metal roof for rainwater collection and uses an incinerating toilet to minimize water usage and avoid the need for sewage disposal, said Carrie Caverly, an architectural designer.

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In practice that generally means dramatically reducing energy consumption through passive design measures and installing a small amount of on-site energy generation, often solar.

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