[Bearleener]

BMW has been experimenting with Peltier elements on the exhaust, I think. Also with exhaust-powered steam turbine generators.

So how much electric power do we need to cruise?
Going back to MetroMPG's original figures: alternator delete improved FE from about 70.68 to 78.08 MPG at 70 km/h.
Converting to metric (sorry, it's much easier for me):
Fuel consumption reduced from to 3.32 to 3.01 l/100km (wow, that's darn good!), i.e. a savings of 0.315 l/100km.
At a speed of 70 km/h we have 0.315*(70/100)= 0.22 l/h
Gasoline has an energy density of about 8.9 kWh/l, so that's 0.22 l/h * 8.9 kWh/l = 1.96 kW. That's how much power the engine is using.
Assuming an alternator efficiency of 55%, an engine efficiency of 29%, the electric power consumption is 1.96 kW * 0.55 * 0.29 = 313 W.
Sounds reasonable (somewhere here I'd seen that keeping the engine running (ECU, spark plugs, fuel pump etc.) takes about 150 to 200 W, so at cruising engine speed it's probably somewhat higher.

So we need to produce that much plus whatever accessories, lights, etc. we've got on.

How about starting the engine? The starter uses about 1 kW, say we crank it for 3 seconds (pretty long), that's 3000 Ws, so if we were to replenish the battery at only 300 W it would only take about 10 seconds (plus a bit for battery losses, probably about 10%). Not a big issue.