Glossary - U.S. Energy Information Administration .
Official Energy Statistics from the U.S. Government. B. Backup fuel: In a central heat pump system, the fuel used in the furnace that takes over the space heating when the outdoor temperature drops below that which is feasible to operate a heat pump. Backup generator: A generator that is used only for test purposes, or in the event of an emergency, such as a shortage of power needed to meet ...
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