Solar is only 1kW per square meter...
Sorry for the old bump....
But keep in mind that solar energy is about 1kW per square meter - close enough to 100W per square foot.
Solar panels may be 25% efficient if pointing at the sun uninhibited. That's 25W per sq'.
Being 3 degrees off the sun is about a 5% output drop, and it drop off quickly when off 5 degrees or more (ie, sun is > 5 degrees normal (perpendicular) to the panel face).
As to "free energy", money-wise, a solar panel typically takes at least 15 years to repay itself. Longer when talking small panels (under say 80W) and including peripheral equipment (batteries, inverters, regulators etc).
Energy-wise - if you tell me how much energy it takes to manufacture & ship a solar panel... well, then you can probably figure it out too.
But there are some good cheap regulators. I use one that costs about USD$20 and handles at least 120W - maybe even 300W (I'd have to check). It isolates the panels from the battery above 14.5V etc.
As to why anyone would want to add solar power to a car for this purpose perplexes me - other than for the fun of it. Solar power to charge hybrid vehicle batteries - yes.
But to charge or power a 90W load, or even less for recharging? Does the power saved from the alternator outweigh the extra gas used in accelerating the panels peripherals?
Granted, the panel payback from gas-generated electricity may be down to 5 years from 15, but still...?
At least nobody with a big sound system would consider this to save gas! Phew!
ramaloji's implementation above is illegal in Australia.
Internal batteries must be sealed else in a sealed enclosure vented top the outside (hydrogen gas liberation, acid spills, etc).
And if ramaloji drives anything like me, those wet cell are gonna splash big time! (Hence why such loads (no pun) should be central else forward in the vehicle.)
Furthermore, ramaloji's parallel battery implementation is asking for trouble. Any cell failure will bring other monoblocks down (equalisation) and eventually failure (through constant discharge to 5/6ths monoblock voltage) and lots of gassing (eg - 5 cells with a 6-cell charging voltage).
Two parallel batteries are bad enough, but four?
Al least being wet (or sealed) lead acid, they are unlikely to experience thermal runaway as would AGM/VRLAs/ Now that's a nice red glow!
Regarding the previously mentioned eeeuser-36643 Solar Panel powered EEE thread.
DO NOT trust Rapture's answers (2nd Reply #27 Here)! EG - Paralleling a 12V car battery & Li-Ion 9V battery is not called "charge equalisation", that is called "EXPLOSION!".
Earlier, both shadyman & zeo mentioned explosions from incorrect charging - imagine connecting to a another (higher voltage) battery which is a much bigger current source!
Charge Equalisation refers to the same batteries connected in parallel if using his analogy, though it generally pertains to charging methods that "equalise" batteries irrespective of series or parallel connection.
As to suggesting the capacitor array (in parallel with batteries!).....
As to Rapture's "V=IR, so lowering the voltage would increase amperage" - if he is talking about transformers or non-linear devices like SMPS-type DC-DC converters and similar constant power loads, then YES. But NOT simple linear voltage regulators and linear loads.
Rapture was responding smadge's earlier inquiry which opens "....if I put the 2 solar panels in parallel.... possibly damaging to any car adapters".
FYI - No it won't. As long as the voltage is acceptable, the load determines how much current it will take.
A car battery won't blow a 12V 250mA bulb (3W) any more than it will a 100W bulb or 1000W amp.