Intensive renovation has made this modest ranch house in the suburbs of Boulder, Colorado, an example of what all green builders strive for — a net zeroProducing as much energy on an annual basis as one consumes on site, usually with renewable energy sources such as photovoltaics or small-scale wind turbines. Calculating net-zero energy can be difficult, particularly in grid-tied renewable energy systems, because of transmission losses in power lines and other considerations. energy home. Changes include an extreme insulation retrofit, a 6kW photovoltaic(PV) Generation of electricity directly from sunlight. A photovoltaic cell has no moving parts; electrons are energized by sunlight and result in current flow. array installed on the roof, and an evacuated-tube solar hot water system.