Energy density measures how much energy is stored in a given amount of space or mass. It is usually expressed in megajoules per kilogram or watt-hours per liter. High energy density means more power in a smaller, lighter package. That matters for cars, planes, and phones.

Fuels differ widely. Gasoline holds about 44 megajoules per kilogram. Coal holds 24 to 35. Wood holds around 16. Lithium-ion batteries hold roughly 0.3 to 0.7 megajoules per kilogram. That gap explains why electric vehicles need large, heavy battery packs to match the range of a gasoline car.

Scientists are pushing battery energy density higher. Solid-state designs promise two to three times the density of today's lithium-ion cells. Lithium-sulfur and lithium-air are further out. Even with progress, liquid fuels will likely dominate aviation and shipping for years. The physics is stubborn. Batteries improve gradually. Fuels have had a head start of a century.

Typical values

• Gasoline: about 44 MJ/kg
• Diesel: about 45 MJ/kg
• Coal: 24 to 35 MJ/kg
• Wood: about 16 MJ/kg
• Lithium-ion: 0.3 to 0.7 MJ/kg