to carry away floating debris. Another solution is to utilize a turbine that resists debris, such as a Gorlov helical turbine or Ossberger turbine. During times of low demand, excess power can be stored in batteries for future use. However, batteries need to be replaced every few years. In many areas, battery expenses can be eliminated by attaching the building to the electric power grid and operating the power system with net metering. Utility permission is required, but such cooperative generation is legally mandated in some areas (for example, California). A grid-based building is less autonomous, but more economical and sustainable with fewer lifestyle sacrifices. In rural areas the grid's cost and impacts can be reduced by using single-wire earth return systems (for example, the MALT-system). In areas that lack access to the grid, battery size can be reduced with a generator to recharge the batteries during energy droughts such as extended fogs. Auxiliary generators are usually run from propane, natural gas, or sometimes diesel. An hour of charging usually provides a day of operation. Modern residential chargers permit the user to set the charging times, so the generator is quiet at night. Some generators automatically test themselves once per week. Recent advances in passively stable magnetic bearings may someday permit inexpensive storage of power in a flywheel in a vacuum. Research groups like Canada's Ballard Power Systems are also working to develop a "regenerative fuel cell", a device that can generate hydrogen and oxygen when power is available, and combine these efficiently when power is needed. Earth batteries tap electric currents in the earth called telluric current. They can be installed anywhere in the ground. They provide only low voltages and current. They were used to power telegraphs in the 19th century. As appliance efficiencies increase, they may become practical. Microbial fuel cells and thermoelectric generators allow electricity to be generated from biomass. The plant can be dried, chopped and converted or burned as a whole, or it can be left alive so that waste saps from the plant can be converted by bacteria. Heating Most autonomous buildings are designed to use insulation, thermal mass and passive solar heating and cooling. Examples of these are trombe walls and other technologies as skylights. Passive solar heating can heat most buildings in even the mild and chilly climates. In colder climates, extra construction costs can be as little as 15% more