energy density (Wh/kg)

Energy density tells you how much energy a storage device crams into each kilogram (gravimetric, Wh/kg) or each litre (volumetric, Wh/L). It is the single number that decides whether a technology can fly, drive, or only sit on the ground. Gasoline is staggering — about 12,000 Wh/kg of chemical energy — while a good Li-ion cell holds only ~250 Wh/kg; even after engines waste most of the fuel's energy, this gap is why batteries are heavy and long-haul electric aviation is so hard.

Crucially, more energy density is not free: pushing ions tighter usually makes cells more reactive and harder to cool, trading safety and cycle life for range. Engineers track a sibling metric, power density (W/kg), which measures how fast you can pour that energy out — a supercapacitor has feeble energy density but enormous power density, while a fuel cell is the reverse. Choosing a storage technology is really choosing where you sit on the energy-versus-power map.