Simple formula is to take the Solar array size and compute an average output of 2.2 times that per day. So, imagine 10kWh Solar system - that's about 22kWh on average. Some days it would be 40kWh but others under 20 if it's rainy, really hot, really cloudy, etc.
Would you consider an on-grid system instead? It will be much more cost effective.
You wouldn't use the power just to recharge the car. It would be to power the house too, right? You need to know your home's kWh usage on a monthly basis now and figure in the additional 12.5kWh per day driven or about 375kWh/month if driven 30days/mo. 375kWh = ?? today on your electric bill. Mine would be 375*.18=$67.50 (but I also don't drive every day, so for me it differs)
An off-grid system of about 10kW (appx. 45 * 235W panels) would do it - but then batteries are the key. I've heard that the 2 Volt batteries are best for off-grid but I think that is for 12v homes (like cabins in the mountains). 120v would be better with 12v or higher voltage batteries. The # of batteries and their capacity is what needs to be looked at. Keep in mind that 1000 50% discharge cycles is about what the batteries are rated for. That is "only" 3 years of recharging the car daily and normal home use, perhaps. If that is how the system is used, it may require new battery sets every 3-4 years. I just don't see the cost-effectiveness of Off-grid solar for our EV and full home use in modern society.
Off-grid is best for standby-power in case the grid goes down. As a "battery backup" when you have Solar PV as your primary power during the day but use grid power at night. Off-grid is also good for very rural homes or mountain cabins. If you have an On-grid solar system, your electric bill can be zero for the year if you size it right. But that's before adding the Volt to the equation.
The problem with battery storage on a daily basis is their wear. I believe that there is technology for Solar farms that will store power in underground salt pits or caverns full of compressed air (a bit like a water reservoir for hydro). But battery chemistry wears out and thus just like the Volt is only warrantying the battery for 100,000 miles (a few thousand recharges) - an Off-grid solar storage system cannot last forever if discharged daily.
http://www.renovusenergy.com/battery_bank_sizing.html
Notice there that 10kW systems would output an average of 28kWh a day - half for the Volt and half for the house. I hear that off-grid systems don't really like to fully-cycle their batteries every day. Just like the Volt, running within a middle-band of discharge is best for longevity of the storage batteries. So "more batteries, the better" to limit the discharge depth. If your daily load on the storage pulls 1/2 the charge normally, that is better than pulling daily 3/4 or 7/8th storage. The reason is that the Sun may not fully recharge the batteries every day. It will be cloudy. You should size the system based on Average solar storage and not maximum.
www.wholesalesolar.com is a good site to look at "parts pricing". Add about 30-40% to such prices for installation by a reputable installer.
I priced out a 6.5 kW on-grid system here in PA. It was $33,500 installed (had another quote of $42k - so your results will vary). That would have gotten me about 8000 kWh of power per year - which is about $1520.00 of local power company costs/year. Minus incentives such as 30% federal and yearly "Solar RECs" which also help pay back the system -- without incentives, you have like a 20 year pay-back for an on-grid solar system. Off grid prices are approximately double that of on-grid due to battery costs, charger cost and a little more maintenance. My payback computation (doing it my way not the installers) for an on-grid was 7.5 years. Of course, it would require financing so that ROI cost goes out to 8.5 to 9 years with the added interest. Installers praise the SRECs and super-high rate of increase of kWh on the grid - but they lie (don't sales folks lie as a rule??

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My guess for you for a well-sized system would have to be at least $50K for a large enough system to go off grid for the house and to recharge the Volt daily. I really think you should do that rather than "just for the car" charging. You can definitely benefit from powering the house on Solar - I don't think the Federal tax credit is available for a utility system (just-for-car) but is for a full-house system.
On-grid discounts for off-peak charging is very good incentive to get a 2-meter setup with the local power company to charge at night at what may be sub .10/kWh.
I learned a lot attending the Pennsylvania Renewable Energy Festival in 2010:
http://www.paenergyfest.com/ There, I learned that "Solar is good but not cost effective". If you want to be green, don't mind the up-front costs and want to deal with selling Solar Credits every year for the next 10+ years (and they may go away) then go for it.
I met a guy there with a Tesla, electric lawn mower and a 14kW home solar system. He has spent over $150K on Solar and Electric things like the Tesla roadster. Total gear-head in terms of going green. Everyone has their incentives and his was getting off foreign oil and being responsible. Plus, seemed to have the resources to do it.
Thinking about it more - you may want to find out if there is a DC-charging option? It would be far more efficient to go DC all the way - the Volt recharges off 110v/220v AC but converts that to DC. It would be great to have a Solar system store in a battery array in your garage and then do a DC charge directly to the Volt (or any EV). Switching from stored DC batteries up to AC and back to DC is very lossy. Staying DC directly would save possibly 1kWh during recharge (that's just a guess).
I'm no expert but I've learned the above by doing some research (yes, homepower.com is well worth the subscription costs). Also ran RC car racing for a few years and that taught me a lot about DC battery charging, LiPO and NiMH batteries and so on. It transfers somewhat nicely to today's EV world.
Bottom line - I wouldn't buy an on-grid PV Solar system unless I can get it installed for under $5 per Watt. That day will come, but just not here yet. I wouldn't buy an off-grid battery-storage solar system for recharging an EV due to the continuous need to buy batteries every 4 years or so. What I would do to go green is what is most easily attainable - conservation.
Get a Kill-a-watt device, find out where your electric usage wastes are and cut back. I've already dropped 800 kWh hours off my monthly bill by doing this and being smart with power consumption. That's enough to recharge a Volt every day.