I thought about that. You can read my ideas here:
http://mysite.wanadoo-members.co.uk/ecotech/socar7.htm
But basically, for a DIY solar powered Prius (
ie solar cells on the roof that top up your battery when it's parked so it always has about 2 miles city range in EV mode) I'd suggest:
1) Put the panels on the roof and bonnet - there's maybe 4 metres squared of easily panelled area.
2) Use SunPower solar cells - might cost about $2000/m2, but are 20.4% efficient, so worth the extra.
3) Only park in the sunshine!
4) Somewhere sunny (say, California), you're looking at a yearly average of 5.5kWh from the sun per m2 per full day. You'll only get 20.4% of that converted to electrical energy, but times four (4m2 panels) means you could get about 4.5kWh electrical into the battery per day.
5) That's a lot more than the OEM battery can store, so you might want to put a second battery in to store it all!
4.5kWh might not sound very much, but at 4 miles per kWh, it's 18 free miles per day which would go a long way to preserving your ICE and could suffice for a lot of people's commutes. At $10k for an install, however, it'll only start paying for itself when gas hits $4.10 / gallon!
Still a nice idea though.... 8)