<P> Off - grid PV systems have traditionally used rechargeable batteries to store excess electricity . With grid - tied systems, excess electricity can be sent to the transmission grid, while standard grid electricity can be used to meet shortfalls . Net metering programs give household systems a credit for any electricity they deliver to the grid . This is handled by' rolling back' the meter whenever the home produces more electricity than it consumes . If the net electricity use is below zero, the utility then rolls over the kilowatt hour credit to the next month . Other approaches involve the use of two meters, to measure electricity consumed vs. electricity produced . This is less common due to the increased installation cost of the second meter . Most standard meters accurately measure in both directions, making a second meter unnecessary . </P> <P> Pumped - storage hydroelectricity stores energy in the form of water pumped when energy is available from a lower elevation reservoir to a higher elevation one . The energy is recovered when demand is high by releasing the water, with the pump becoming a hydroelectric power generator . </P> <P> Beginning with the surge in coal use which accompanied the Industrial Revolution, energy consumption has steadily transitioned from wood and biomass to fossil fuels . The early development of solar technologies starting in the 1860s was driven by an expectation that coal would soon become scarce . However, development of solar technologies stagnated in the early 20th century in the face of the increasing availability, economy, and utility of coal and petroleum . </P> <P> The 1973 oil embargo and 1979 energy crisis caused a reorganization of energy policies around the world and brought renewed attention to developing solar technologies . Deployment strategies focused on incentive programs such as the Federal Photovoltaic Utilization Program in the U.S. and the Sunshine Program in Japan . Other efforts included the formation of research facilities in the U.S. (SERI, now NREL), Japan (NEDO), and Germany (Fraunhofer Institute for Solar Energy Systems ISE). </P>

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