A flywheel is a heavy wheel attached to a rotating shaft. Expending energy can make the wheel turn faster. This energy can be extracted by attaching the wheel to an electrical generator, which uses electromag.
[pdf] Flywheel energy storage systems are highly efficient, with energy conversion efficiencies ranging from 70% to 90%. However, the efficiency of a flywheel system can be affected by friction loss and other energy losses, such as those caused by the generator or motor.
[pdf] Most modern inverters have efficiency ratings between 90% and 98%. Let’s break it down: If you feed 1000 watts of DC power into your inverter and it outputs 950 watts of AC power, your inverter efficiency is 95%. The other 50 watts were lost in the conversion process—mostly as heat.
[pdf] 
