flywheels solve a narrow problem and are worse than batteries at delivering long term energy. The examples linked in other comments are around 10 kWh of energy. Most north american houses consume around 30-80 kWh per day. The real problem with flywheels is scale and longevity. How many flywheels can you name that have been in service for 20 years? Lithium batteries have won the short duration energy storage race and most other solutions are for edge cases.