energydensity

Energy density, sometimes written energydensity, is the amount of energy stored in a substance or system per unit of mass or per unit of volume. It is typically described in two forms: gravimetric energy density (per unit mass) and volumetric energy density (per unit volume). Common units include joules per kilogram (J/kg) or watt-hours per kilogram (Wh/kg) for mass-based density, and joules per liter (J/L) or watt-hours per liter (Wh/L) for volume-based density.

Measurement and context vary by application. For chemical fuels, energy content is determined by measuring the usable energy released during combustion. For electrical energy storage devices, such as batteries, energy density reflects how much energy can be drawn per kilogram or per liter during discharge. In nutrition, energy density describes calories per unit mass of food and is used to compare the energy content of different foods.

Fuels and combustion-based systems typically have high gravimetric energy density. Gasoline is about 44–46 MJ/kg, diesel similar, and ethanol somewhat lower (~30–34 MJ/kg). Volumetric values are also important for fuels; gasoline is roughly 34 MJ/L. Batteries generally have lower gravimetric energy density than hydrocarbons but are crucial for portable power; lithium-ion cells commonly range from about 150–250 Wh/kg (roughly 540–900 Wh/L, varying with design). New chemistries and solid-state designs aim to increase both gravimetric and volumetric densities.

In foods, energy density is expressed as calories per gram; fats provide about 9 kcal/g, while carbohydrates and proteins provide about 4 kcal/g, with water-rich foods offering much lower energy density. Higher energy density foods deliver more calories per unit mass but may differ in nutrient content.

Applications of energy density metrics include engineering design, performance optimization, and dietary planning, where trade-offs among energy availability, weight, volume, safety, and cost are considered.