<P> Since the early 21st century batteries have been applied to utility scale load - leveling and frequency regulation capabilities . </P> <P> In vehicle - to - grid storage, electric vehicles that are plugged into the energy grid can deliver stored electrical energy from their batteries into the grid when needed . </P> <P> Chemical fossil fuels (gas, oil, coal) remain the dominant form of energy storage for electricity generation, within natural gas becoming increasingly important . </P> <P> Thermal energy storage (TES) can be used for air conditioning . It is most widely used for cooling single large buildings and / or groups of smaller buildings . Commercial air conditioning systems are the biggest contributors to peak electrical loads . In 2009, thermal storage was used in over 3,300 buildings in over 35 countries . It works by creating ice at night and using the ice to for cooling during the hotter daytime periods . </P>

What do cells use for energy storage and as a source of energy