below the earth, temperature ranges from in North Dakota to , in Southern Florida. Wind breaks reduce the amount of heat carried away from a building. Rounded, aerodynamic buildings also lose less heat. An increasing number of commercial buildings use a combined cycle with cogeneration to provide heating, often water heating, from the output of a natural gas reciprocating engine, gas turbine or stirling electric generator. Houses designed to cope with interruptions in civil services generally incorporate a wood stove, or heat and power from diesel fuel or bottled gas, regardless of their other heating mechanisms. Electric heaters and electric stoves may provide pollution-free heat (depending on the power source), but use large amounts of electricity. If enough electricity is provided by solar panels, wind turbines, or other means, then electric heaters and stoves become a practical autonomous design. Water heating Hot water heat recycling units recover heat from water drain lines. They increase a building's autonomy by decreasing the heat or fuel used to heat water. They are attractive because they have no lifestyle changes. Current practical, comfortable domestic water-heating systems combine a solar preheating system with a thermostatic gas-powered flow-through heater, so that the temperature of the water is consistent, and the amount is unlimited. This reduces life-style impacts at some cost in autonomy. Solar water heaters can save large amounts of fuel. Also, small changes in lifestyle, such as doing laundry, dishes and bathing on sunny days, can greatly increase their efficiency. Pure solar heaters are especially useful for laundries, swimming pools and external baths, because these can be scheduled for use on sunny days. The basic trick in a solar water heating system is to use a well-insulated holding tank. Some systems are vacuum- insulated, acting something like large thermos bottles. The tank is filled with hot water on sunny days, and made available at all times. Unlike a conventional tank water heater, the tank is filled only when there is sunlight. Good storage makes a smaller, higher-technology collector feasible. Such collectors can use relatively exotic technologies, such as vacuum insulation, and reflective concentration of sunlight. Cogeneration systems produce hot water from waste heat. They usually get the heat from the exhaust of a generator or fuel cell. Heat recycling, cogeneration and solar pre-heating can save 50–75% of the gas otherwise used. Also, some combinations provide redundant reliability by having several sources of heat. Some authorities