Thanks ctgottapee. That's pretty much as I imagined re the flexible solar panels, but there's no substitute for getting the gen from someone with first-hand experience. They look like quality gear but I'm not planning on sticking any on my car roof just yet.
I was thinking of getting a pair of 20w 'semi-flexible' panels...
(like these...
Renytek. Semi Flexible Solar Panels ...although they don't seem to have the 20w ones on their web site at the moment)
...suspending them in the two rear-most side windows in the load area of my Mk4 Golf estate (wagon), and using them to top-up the charge on a battery which is recharged daily from the mains and almost never charged by an alternator. They wouldn't be hugely efficient I guess as they will be almost vertical, and will be behind glass, but those rear windows serve no function otherwise as the driver can't see out of them, so I may as well use the space for something useful.
If they're mounted inside the vehicle then there would be no aero drag from them, ...and of course they are very lightweight. I'm hoping that 40w of panel would deliver at least an amp or so under most daylight conditions. Even one amp or solar would go a long way towards extending my alternator-free driving range, especially over, say a long weekend, where plug-in charging is not possible at a destination but where the car is used just to get there and to get back, ...which is quite typical usage for me on a camping trip, for example.
For day-to-day driving, for work and so on, charging from the mains power supply is proving more than adequate, but even under those conditions a solar panel would reduce the overall daily Depth of Discharge and extend the life of the batteries to some extent.
From your experience of solar panels, do you think that might be worth doing? Do you think one amp of current from 2x 20w panels is a realistic expectation?
By the way, I have been getting mpg consistently at least 10% higher since I started plugging in every night. Well, nearer 15% higher in fact, but I put some of that down to the 2kW engine coolant pre-heater being switched on for half an hour before driving off every morning. Whenever I've done tests I've found a good 10% mpg boost from not using the alternator. Obviously that figure will be greater as a percentage with an already efficient vehicle like mine, as an efficient vehicle will already be using a greater percentage of fuel to produce electricity, but the actual amount of fuel saved per hour will be about the same in any vehicle (with the normal level of electrical equipment on board.)