are dispatched only when necessary, generally when energy demand peaks. This is called "economic dispatch". Demand for electricity from the world's various grids varies over the course of the day and from season to season. For the most part, variation in electric demand is met by varying the amount of electrical energy supplied from primary sources. Increasingly, however, operators are storing lower-cost energy produced at night, then releasing it to the grid during the peak periods of the day when it is more valuable. In areas where hydroelectric dams exist, release can be delayed until demand is greater; this form of storage is common and can make use of existing reservoirs. This is not storing "surplus" energy produced elsewhere, but the net effect is the same – although without the efficiency losses. Renewable supplies with variable production, like wind and solar power, tend to increase the net variation in electric load, increasing the opportunity for grid energy storage. It may be more economical to find an alternative market for unused electricity, rather than try and store it. High Voltage Direct Current allows for transmission of electricity, losing only 3% per 1000 km. The United States Department of Energy's International Energy Storage Database provides a free list of grid energy storage projects, many of which show funding sources and amounts. Load leveling The demand for electricity from consumers and industry is constantly changing, broadly within the following categories: Seasonal (during dark winters more electric lighting and heating is required, while in other climates hot weather boosts the requirement for air conditioning) Weekly (most industry closes at the weekend, lowering demand) Daily (such as the morning peak as offices open and air conditioners get switched on) Hourly (one method for estimating television viewing figures in the United Kingdom is to measure the power spikes during advertisement breaks or after programmes when viewers go to switch a kettle on) Transient (fluctuations due to individual's actions, differences in power transmission efficiency and other small factors that need to be accounted for) There are currently three main methods for dealing with changing demand: Electrical devices generally having a working voltage range that they require, commonly 110–120 V or 220–240 V. Minor variations in load are automatically smoothed by slight variations in the voltage available across the system. Power plants can be run below their normal output, with the facility to increase the amount they generate