It might help if you think in terms of watts (or kW) instead of horsepower (hp). They are the same unit (power, or derivative-of-energy-over-time) but "aimed at different people" as it were: kW is for electricity, hp is for motors. As it happens, there's also a scaling ratio (of about 4/3 or 3/4) depending on which way you go, so that 400 hp is about 300 kW and vice versa.

Now, the electric motors can draw 300 kW (of electric power, to produce 400 hp of "motor makes car go"). (It's never been clear to me how much heat loss there is in this process. 300.000 kW is 402.307 hp, to be more precise, but any energy lost as heat means that to get 402.307 hp "out" you have to put

*more* than 300 kW "in"; or, equivalently, if you put 300 kW "in" you might get, say, 400 hp "out" and roughly 2.3 hp worth of heat. But let's ignore losses for now, since we hope they are small enough to ignore.)

Anyway, the 300 kW is, as you note above, a

*maximum* (sustained anyway) power draw. Most of the time, one uses far less energy than 300 kWh per second (remember, energy is the integral of power so we have to measure the power draw for some time, like one second at a time, and talk about "energy used per unit time") to sustain speed. It's only when climbing a big hill or, as in the case of the Veyron someone mentioned elsethread, going 250 mph through air that at that speed seems more like treacle / molasses, that you need huge amounts of power for long sustained periods. (Incidentally "air drag" is a terribly complicated calculation because there are laminar and turbulent flows and you need to know the "Reynolds number" and look at turbulence boundaries and so on, but there's an approximation using "coefficient of drag" and the area of the car that's usually good enough, that uses square of velocity, so that 250 mph is four times more drag than 125 mph. Using this rule, and ignoring coefficients of drag and car surface areas, if the Veyron requires ~1000 hp to overcome 4x the drag force, the Karma should be able to sustain 125 mph with a mere ~250 hp, or about 185 kW.)

Now, the motor-generator in the Karma produces roughly 175 kW (from memory—you said "260 hp" which is closer to 190 kW). At a sustained 75 mph on an overall-flat-road, the Karma should need roughly 140 kW of electric drive (using the numbers from the earlier parenthetical aside).

*If* the generator cranks out the full 175 (190?) kW, then all the "extra" electrical power can go into recharging the batteries. That should be at least 35 kW (175 - 140 = 35). If you were to sustain that for one hour, that would produce 35 kWh, which is more than the battery holds (roughly 20 kWh).

Using all the above numbers (which are all theoretical rather than measured), it should be possible to drive the Karma 75 mph for about 40 minutes and still go from "empty battery" to "full battery". However, this would mean running the gasoline engine pretty close to flat-out for that entire time. Turn the engine output down a bit (and/or account for various system losses) and you need more time to recharge, or you have to go slower than that.

(I had a bunch of interruptions writing this, and have another thing to get to now, so I hope there are no major glitches above.

)