Energy density
Energy density is the amount of energy stored per unit mass or volume. It distinguishes gravimetric energy density from volumetric energy density.
Specific energy: Energy per unit mass, commonly expressed in joules per kilogram or watt-hours per kilogram. This is the mass-based measure used to compare how much energy a fuel or battery carries for its weight.
Gravimetric energy density: Energy stored per unit mass of a material or system. This isolates the weight constraint that matters in aircraft, vehicles, and portable equipment.
Power density: Power delivered or stored per unit mass or volume. Energy density describes how much energy is stored; power density describes how quickly it can be delivered.
Electric vehicle range: The distance an electric vehicle can travel before its battery requires recharging. Battery energy density affects how much driving range fits within limits on vehicle mass and space.
Volumetric energy density: Energy stored per unit volume, commonly expressed in joules per cubic metre or watt-hours per litre. It captures the space needed to store a given amount of energy.
Energy density of fuels: The energy obtainable from a fuel per unit mass or volume, usually through combustion. Fuel composition and the chosen mass or volume basis determine the reported value.
Energy-to-weight ratio: A comparison of energy content with the weight of a fuel, device, or system. This informal ratio can obscure whether energy is normalized by mass or by gravitational force.
Aircraft weight: The total mass of an aircraft, including its structure, payload, fuel, and equipment. Low fuel energy density can impose severe weight penalties on long-distance electric flight.
Lithium-ion battery: A rechargeable battery that moves lithium ions between electrodes during charging and discharging. Its energy density shapes the range, weight, and size of many portable devices and electric vehicles.
Thermodynamic efficiency: The ratio of useful energy or work obtained from a process to the energy supplied. Stored energy density does not by itself determine how much energy a device can usefully deliver.