It's not X kWh of batteries for X kWh of flywheels, it's 0.01 kWh of batteries to provide enough power to run the flywheel's protection and control systems while it 'freewheels'. Ideally, you don't want to brake the flywheel while the grid is down, because then you're losing all the storage energy and need several minutes to spin back up when the grid is back.