If you are visiting Penzance by car, parking shapes the day more than almost anything else. The useful question is not simply where the car can go, but where it can sit safely while you walk the town without constantly moving it.
A good parking choice should keep the harbour, Chapel Street, the promenade or the town centre within easy reach. In summer especially, it is worth arriving earlier than you think you need to so you are not circling when you would rather be on foot.
Once the car is parked, the rest of the day becomes much easier. Build the visit around one area at a time and you will spend more time enjoying Penzance than watching the clock.