Beacon Power, a flywheel manufacturer has just extended the amount of power flywheels can store to 20 megawatts.

The benchmark for flywheels though - on power storage - has to be measured in watt hours per kilo. According to this wiki, they are at a still low 130 watt hours per kilo which compares to 160 for a lithium ion battery.

Flywheels then although they don't have the energy density of lithium ion batteries, do have much greater scaleability.