Flywheel technology is not meant as a power backup, it is meant for frequency regulation which fluctuates on a second by second basis. Without power grid frequency regulation you would have brown outs and destructive damage to electrical equipment.
Power grid frequency is kept within +/- 0.5Hz of 60Hz. In times of high demand (during the day) inefficient (~50%) and polluting diesel generators are kept running to meet the demands.
As stated, power plants can't respond in seconds but take many minutes to adjust to demands. Flywheel technology siphons off excess energy created during low demand time and supplies it back to the grid in times of high demand with no diesel and no pollution.
"I tremble for my country when I reflect that God is just." - Thomas Jefferson
