Energy density

The energy a battery or fuel stores per kilogram (specific energy) or per litre.

• The best lithium-ion cells hold about 250–300 Wh/kg; jet fuel about 12,000 Wh/kg, roughly 40–50 times more.
• Electric drives turn about 90% of stored energy into work versus roughly 35–40% for jet engines, narrowing the effective gap to about 15–20×.
• Batteries also don't get lighter as they empty, while planes burn off fuel, which is why battery aircraft suit short flights.
• Pack figures run roughly 20–40% below cell figures once casing, wiring and cooling are added, so check which one is quoted.