Abstract: The present invention includes a mechanical energy storage device and method of making and using the same comprising: two or more pneumatic or hydraulic capacitors or accumulators, each of them connected to at least one hydraulic or pneumatic exhaust manifold and a hydraulic or pneumatic intake manifold through exhaust and intake valves, respectively; at least one hydraulic fluid or pneumatic reservoir in fluid communication with the hydraulic or pneumatic exhaust manifold via a hydraulic or pneumatic motor connected to an output device, and in fluid communication with the hydraulic or pneumatic intake manifold via hydraulic pump or pneumatic compressor driven by a source of variable power; and a governor or control valve disposed between the hydraulic or pneumatic exhaust manifold and the hydraulic or pneumatic motor connected to the output device. The use of compressible gas, pneumatic, and air are interchangeable for the purposes of this device.