These are measures of lost or retained capacity (Ah), respectively, while in a stored state. Energy loss and energy retention are the counterparts of self-discharge and capacity retention, except that they measure retained energy (Wh) rather than capacity. While capacity-based metrics are more commonly reported, likely due to ease of calculation, energy-based metrics may be more relevant to applications where a constant power, rather than a constant current is drawn from the power source. In this paper, energy retention is used for reasons discussed in (Section 3.5). Energy retention can be expressed in terms of relative energy retention per unit time (% year -1 ) or in terms of absolute energy retention per unit time (Wh year -1 ). Calendar life is a related measure of energy retention that can be expressed as the time before energy retention drops below 20% of its initial amount. Energy retention is important in applications where the battery is not in constant use-such as with a parked electric vehicle , long-term space technologies , medical devices , and low-power devices .