I have seen some of them for 9WATTS@12VDC output for a cheap cost... Thats not a whole lots of amps to use for much of anything... Thats around .7 AMPs and only when you are in direct sunlight... I think I have one of those panels around here somewhere...
I used to plug one into a cigarette lighter on my Jeep to trickle charge the Jeep battery but the it seems the newer vehicles like to switch off the 12V plug when the ignition key is turned off after a short time out.
Too me its is much better to just use my 600WATT PSW Inverter 120VAC power source that is always 'ON' connected to my trailer battery bank.
I have two locations in my OFF-ROAD Trailer where I have run an out-of-sight 120VAC extension cord with multiple 120VAC sockets from the two 120VAC connection on the Inverter.

I always re-charge my battery bank each morning at 8AM for three hours using my 2KW Generator (when allowed) connected to the trailer shore power cable when camping off the power grid... This is when I make up my good fresh ground bean coffee for the day...
Might save the day for something so is a good PLAN B thing I reckon. I wouldn't spend alot of money for one however...
Roy Ken