Solar Buildings - Another approach to the design of sustainable buildings is to install on buildings either passive solar systems to reduce the energy needs of a building or active solar systems such as solar hot water systems or solar photovoltaic panels to produce electricity. Solar tubes that reflect and channel sunlight to where it is needed in a building is a new method of passive solar design. Meanwhile, a single solar photovoltaic panel can typically produce about 180 watts of electricity. Hence, a typical array of 24 solar panels will produce about 4,000 watts (i.e. 4 KW) of electricity on a bright sunny day. Inverters take the electricity from the solar panels and convert them from DC current to AC current. This current is then used to provide the electricity needs of a building. Under federal law, any excess electricity has to be bought by the local electrical utility. Alternatively, an excess electricity generated can be stored in batteries and used later. Since the average home in the U.S. uses enough electricity to generate approximately 10,500 lbs. of carbon dioxide emissions, reducing the electricity bought from the local utility produced by conventional fossil fuels can result in significant reductions in a home or building's carbon usage (i.e. footprint). While the cost of solar panels on a home can cost anywhere from $10,000-$50,000, the cost can be recovered over time through reduced utility bills. Also, many states offer tax credits or deductions for the installation of renewable energy sources such as solar panels on a building. Finally, there are a variety of methods being developed to concentrate and thus increase the amount of solar energy capture from solar energy systems. A simple way to do so is to use mylar to reflect additional sunlight onto the panels. Using this method, one can often produce ten times the electricity of normal solar panels. Above is a photograph of the K2 sustainable apartments in Windsor, Victoria, Australia that are built with not only both passive solar and solar photovoltaic systems, but also are made of recycled materials and include rainwater collection on the roofs.