In most applications, scavenged energy is not always available. With solar, the energy source is lost at night. In an asset tracking system, which relies on truck vibrations, the energy source may be highly irregular. Vibrations will disappear when the truck stops. Therefore, an energy reservoir may be needed to power the system during these outages. The harvester may also not be able to provide the peak power requirements of the system. It may be able to supply the average power requirements, but for peak power, again, a reservoir will be necessary. The storage element acts as an energy buffer to the system. Proper selection of an energy storage device to meet the peak power requirements of the system can prevent over-sizing of the energy harvester. Ideally, all of the energy from a harvester could be captured for later use, so the storage technology should be rechargeable. Another necessary attribute of the storage technology is that it have ultra-low leakage or low self-discharge.