where there is not enough hydroelectricity to respond to daily and weekly variations in generation and consumption. It is not unusual for a dam to be built with more capacity than can be sustained by the water supply, allowing for a higher peak output. Upgrading equipment at existing dams can be one of the least expensive ways of increasing peak generation. The ability to vary the amount of electricity generated is often limited by the requirement that minimum or maximum flows downstream are satisfied. Pumped-storage hydroelectricity is the largest-capacity form of grid energy storage available, used for averaging off-peak and peak electrical demands. The site stores energy using the gravitational potential of water stored in a reservoir. Low-cost off-peak electric power from base load or intermittent sources is used to pump water at a low elevation to storage in a high elevation reservoir. During periods of high electrical demand, the stored water is released through turbines to produce electric power. Start up times are only a few minutes, and some can start in a few tens of seconds. Batteries are used in some cases where conditions favor it to smooth flow (avoiding a costly power line upgrade) as well as supplying peak power and other grid services such as operating reserve, sometimes in hybrid configuration with turbines or diesel engines. Battery power is by far the fastest responding of all powerplants, and can respond to grid conditions at millisecond timescales, giving slower responding equipment a chance to react to outages. Pumped-storage and batteries are net consumers, as they have no inherent energy source, and the conversion between electricity and storage and back incurs some losses. Solar thermal peaker plants were proposed in 2017, under a US Department of Energy Technology 2 Market award to Hank Price of SolarDynamics, whose paper "Dispatchable Solar Power Plant" proposed utilizing the thermal energy storage inherent in a solar thermal energy power plant, that enables this heat-based form of solar to generate like a gas peaker, to supply power on demand day or night, and in return be controlled by the utility and paid in capacity payments to be available when needed, like a traditional peaker plant. A solar thermal power plant makes electricity in a steam cycle power plant like a traditional power plant but the heat for steam is supplied by solar energy heating a material such as molten salts and storing the