Energy storage facilities become essential when a considerable amount of renewable energies is installed. However, an economic energy storage technology, which aims stabilization of several hours to several days, is not existing. Electric thermal energy storage is proposed to fill this technological gap. The electric thermal energy storage consists of the following processes. Electricity is converted to heat, and the heat is stabilized in a thermal energy storage system. The thermal energy storage produces electricity whenever required using a thermal cycle. Although conversion efficiency from the thermal energy to electricity is low, the nature of the very low-cost thermal energy storage overcomes the low efficiency from an economic viewpoint. Several developing projects of the electric thermal energy storage are undergoing. None of them reveals a quantitative analysis of the economy yet. Two case studies employing simplified assumptions are conducted to prove the economy of electric thermal energy storage. The electric thermal energy storage generation cost with one-week energy storage becomes 15 cents/kWh when a renewable generation cost falls to 2.5 cents/kWh in 2030 using existing technology. Nine cents/kWh, which is competitive energy cost, is expected when a combined heat and power application or thermal to electricity efficiency is improved. The electric thermal energy storage employing a synchronous rotating heater has many advantages. The synchronous rotating heater realizes cheaper electric thermal energy storage and has a synchronous inertia and an ability to produce failure current to activate a grid protection system. This ability eliminates an auxiliary stabilizing system for a grid such as rotating condenser and development of a new grid protection system. (C) 2019 Elsevier Ltd. All rights reserved.