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What a great thread! That's a MASSIVE battery, and a MASSIVE solar panel. I once calculated that if I had about 200w of solar panel then that would pretty much negate the need for an alternator, assuming a large enough storage battery.
The aerodynamic hit from having a large, square panel on the roof worries me though. I came to the conclusion for myself that at some point I would build some raw solar cells into the paint layer, maintaining the original profile and covering them with a smooth layer of resin, feeding all the wires through the roof and fitting all the connectors, diodes, etc. inside the car, ...but life is short. That is though what I believe manufacturers could be doing at a relatively modest cost. Solar cells will last the life of a vehicle and would look pretty cool. Roof panels (and bonnets) rarely get significant minor accident damage in day-to-day driving, so those (hypothetical) factory panels should keep on working til the car is scrapped.
I am less worried about the effect of the extra weight. An additional 50kg (your battery must weigh at least that!) is only 3% of the weight of a 1500kg car, and so the maximum possible mpg hit from that is 3% - and that is if you are using all your fuel to climb hills slowly, never coming downhill again, and at speeds at which air resistance is negligeable, which as far as I know would mean well below 30mph, although that will depend on the shape of the car. As far as additional rolling resistance goes, I think that if the tyres are pumped up nice and hard to account for the extra weight (as per manufacturer's instructions, by the way, although I guess we all run tyre pressures way higher around here anyway) then rolling resistance shouldn't be affected too much at all, especially if the weight distribution is arranged so that no one tyre or pair of tyres is overloaded.
As for adjusting alternator output to accomodate AGM batteries' higher voltages, well I just fried my alternator by trying to fit a switch to turn it off with the engine still running (by grounding the field current) so perhaps you should be wary of anything I say, but I've heard, and I believe, that a simple diode inline with the alternator's VOLTAGE SENSE wire (teh one that goes direct to the battery, bypassing the larger high current wire) would fool the alternator into 'thinking' the battery was half a volt (or less or more - choose your diode carefully) lower in voltage than it really is, and so it would put out a half volt more. (Schottky diodes have a lower forward voltage drop than standard ones.) Getting an alternator to put out a LOWER voltage is difficult but a slightly higher voltage should be pretty easy. Having said that, I run an AGM battery and my preference would be to run the alternator at a lower voltage, so that it runs the car's electrics when the battery is low but doesn't 'waste fuel' trying to fully recharge a partially discharged battery. I would rather recharge the battery fully at my destination using mains power.
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