I have the AA 2.4 amp version. I have only used it once on a Mk4, which has a comparatively small 12v battery, but it only needs to power up the cars electrics , not crank a starter motor to start the engine.
It was connected for 2 months in January and February .No problem starting the car on my return, but as Colin B points out, I have no way of knowing if it might have started anyway without the solar 'charger' . And I agree the best you can hope for is it will replace some, or most of ,the normal 'parasitic loss.'
I attach a copy of the instructions which you may find interesting. There are two more pages which add little except that the peak power output is 2.4 watt, 12.7 volts and optimum working current is 0.13 -0.2 amps. (130 - 200 milliamps - I think. I am not good with small numbers

)) I assume 'optimum' means in good sunlight.
Modern cars normally have constant parasitic drain of 50 - 85 milliamps due to alarm systems etc running 24/7. So with long summer days and good sunlight the 2.4 watt panel would probably keep ahead of battery drain.
But what of short winter days, with less light? I have just run an experiment. Its currently (excuse pun) quite bright, with occasionally enough sun to cast a shadow. Indoors the panel was giving about 2 milliamps away from the window, 5 milliamps close to the window,.( Bright but no direct sun. ) When the sun cast a shadow these figures increased to about 5 milliamps and 12 milliamps. Outside ,in direct sun , it was about 20 milliamps. (The intensity of light made a big difference and I have no reason to doubt the panel could output up to 200 milliamps in summer sun. )
2.4 watts would probably be enough in summer, but might not manage in winter. But If the battery is in good condition and fully charged at the start it might give just enough extra charge to tip the balance in your favour.
One fear I had was that extended unattended use in bright summer sun could overcharge the battery. . The 2.4 watt version claims this wont happen but I dont know about the 4.5 watt version.