Thermal Energy Storage (TES) is a technology that stores "cooling" energy in a thermal storage mass. In the eighties and early nineties the utilities in California incentivised this technology to shift electrical on-peak power to off-peak. Thereafter, for various reasons TES became the most neglected permanent load shifting opportunity. It is only now with the challenges that the renewables provide that TES may have a come- back because it is basically the best and most economical AC battery available with a round trip efficiency of 100% or even better. This presentation gives some background to this development and shows the interdependence of TES to the tariffs. An example is given showing the challenges and experience required to design a feasible project with today's tariffs. "The first law of energy storage dynamics" is then developed to achieve the smart grid.