This is about 20% less than the rule of thumb number for the power needed to keep a modern mid size car moving at about 100km per hour. I forget where I read it.

So yes pretty good.

And this is the principal reason why internal combustion engines are so inefficient in practice, they are used at a tiny fraction of their rated output for most of their working lives.

Of course the comparison is not quite as favourable as it at first appears because we should really measure the input power not at the output of the battery but at the input of the battery charger (equivalent to measuring the fuel pumped into a petrol engine's tank). Still impressive even if the conversion efficiency of the charger and battery combined is only 50%.

Cool, do you have a source for that rule of thumb? That's a new one for me. That's about 8HP to the wheels to keep it rolling at 100km/h right? So obviously more than that from the engine.

Re: measuring at battery input rather than output, that's a good point. Would definitely be higher that 50% charging efficiency though. I would guess at least 80% - probably > 90% if you charge it slowly enough. (Haven't done any research on Tesla chargers or batteries specifically, but I did used to be a power electronics engineer. High charging efficiency is important both for cost and, perhaps as importantly, because a slight reduction in efficiency can mean a lot of extra heat that you have to deal with.)

Alas i can't remember and a quick web search doesn't find anything I recognize. But I did find this that mentions 10hp at 50mph: http://energy.typepad.com/the-energy-blog/2010/02/energy-nee... And a few hotrodders discussing the question here: http://www.hotrodders.com/forum/how-much-hp-required-60mph-1... This on PhysicsForums: https://www.physicsforums.com/threads/power-and-tq-needed-to...

And finally a paper with real formulas and better explanations: http://wps.aw.com/wps/media/objects/877/898586/topics/topic0...

Looks like my 5kW wasn't too far off.