So if you really had an interest on this topic, I ran the numbers on how Elon came up with his 400 Wh/kg on the Electric Airplane thread, and if you are curious to see what kind of design compromises are required:
Electric planes
Look, I am Elon's #1 fan (on equal ranking with many others I suspect) and all I have been doing is clean sheet aircraft designs and out of the box thinking, which even got me fired from Textron at one point (pretty proud of that) because I was too disruptive.