**1.5 Energy storage technologies**

The two choices available for energy storage are batteries and electrochemical double layer capacitors, known as ultracapacitors. Batteries are a relatively mature technology and have a higher energy density (more capacity for a given volume/weight) than ultracapacitors, but ultracapacitors have a higher power density than batteries and have traditionally been used to handle short duration power surges.

Recently, such capacitors have been explored for energy storage, since they are more efficient than batteries and offer higher lifetime in terms of charge-discharge cycles. However, they involve leakage (intrinsic and due to parasitic paths in the external circuitry), which precludes their use for long-term energy storage. While it is also possible to implement energy storage mechanism using an ultracapacitor and a battery, it is a tradeoff to a decrease in harvesting circuit efficiency due to the increased overhead cost of energy storage management.
