Grid energy storage refers to the methods used to store electrical energy within an electrical power grid. Electrical energy is stored during times when production (from power plants) exceeds consumption and the stores are utilized at times when consumption exceeds production. In this way, electricity production must not be so drastically scaled up and down to meet momentary consumption instead, production is maintained at a more constant level. This has the advantage that fuel-based power plants (i.e. coal, oil, gas) can be more efficiently and easily operated at constant production levels. In particular, the use of grid-connected intermittent energy sources such as photovoltaics and wind turbines can benefit from grid energy storage. Intermittent energy sources are by nature unpredictable the amount of electrical energy they produce varies over the time and depends heavily on random factors such as the weather.