A downhole kinetic energy storage system for wellbore completions is configured for installation downhole for extended periods of time, such as 10 year or more. The kinetic energy storage system receives power from a low power source, which can be due to a “power bottleneck” to the downhole location such as inductive coupling, optical fiber, downhole energy harvesting, and/or subsea wellhead configurations. The system stores the available low-power as rotational energy in a flywheel and then when demanded converts the rotational energy into electrical energy at a temporary power level exceeding the low power source. The temporary high power energy is used for wellbore completion applications such as actuating a flow control or other downhole valve.