The author believes that batteries as a power source can be categorized by energy destinations with performance requirements (e.g. power) and material/processing limitations (e.g. mass) (Figure , Table ). As a simple exercise, a recreational drone may require 1 kW of power (performance requirement) to generate adequate thrust for lift for a total mass of 3 kg which includes a maximum 2 kg battery (material/processing limitation). A more sophisticated approach involving optimization could be applied such as maximizing flight time and allowing power, mass, cost, etc. to vary or be subject to constraints in which case the performance and material/processing metrics are changing until an algorithm end-condition is met. Performance and materials/processing metrics result in several commonly cited technical criteria for batteries. In the case of the recreational drone example mentioned, one metric would be power divided by mass, or specific power (W kg -1 ). Given the above specifications, the drone would require a battery with a specific power of at least 500 W kg -1 to sustain lift. If the drone was specified to fly for thirty minutes at 1 kW of power consumption, the energy required would be 500 kWh, resulting in a specific energy requirement of 250 Wh kg -1 for the battery. Other requirements such as maximum cost or volume could be imposed as well.