Electricity must be generated as it’s being used, so that demand and supply are always kept in balance.

This is a critical factor with respect to unreliable, wind and solar. When the wind stops blowing or the sun stops shining, supply is cut off from these sources and another source must immediately be brought on line. Typically, the back-up source is a natural gas turbine that has been spinning, i.e., running, using natural gas, but not connected to the grid, which can be quickly brought on line. Using coal-fired power plants for back-up requires them to operate as load following units, cycling them up and down, which creates major thermal stresses on components, and increasing maintenance costs.

Standby power adds to the cost of wind and solar, but usually isn’t included when calculating their cost.

Storage is essential for widespread use of wind and solar, if large quantities of back-up…