Re: Reserve power?
Batteries have nowhere near the energy density of gasoline much less fuel oils.
Lithium-Air batteries have a theoretical specific power of 11.4 kW/kg, compared to the theoretical maximum of 11.99 kWh/kg for Jet A-1, so very close.
We don't have theoretical batteries, you may say, but that's fine, because we don't have theoretical engines with 100% conversion efficiency for jet fuel, either. And if we did, they wouldn't be zero mass and zero volume, so would negatively affect those numbers. And that's without mentioning that jet fuel needs things like storage tanks, pumps, which also add mass and volume.
Your argument sounded less foolish years ago, before electric vehicles like the Teslas came along and proved it's possible to get the same range with batteries as with gasoline for only modestly more weight. Battery technology is not static, manufacturers continue to increase energy densities. In time, they absolutely will become competitive, it's just a gradual process.