Actually, despite what most people think and also some saying the A/C is designed for demisting, I don't think the A/C is actually best for demisting.
If we think of the bathroom mirror effect, the reason it steams up is because the mirror is cold. So steam will condense on it. Same as our windscreens.
When we use A/C on our windscreens, we are doing several things. It makes the water vapour near the windscreen colder so it doesn’t condense as easily, circulates the air and makes the windscreen cold. That’s all good and well if you don’t mind having the A/C on all the time because as soon as you turn it off, it will be far worse than it was before. This is because the windscreen is a lot colder now so it’s even more prone to steaming up! But usually we have this problem when the weather is cold so you wouldn’t really want the A/C on!
IME you’re better off using the heater.
It takes a little longer to demist the windscreen but it will work better for longer and you won’t freeze inside the car! You want to use the heater to heat up the windscreen, evaporate the water and warm up the inside of the car. If you keep the heater blowing at the windscreen it will keep it warm so water molecules can’t condense on it. But as soon as all the water has evaporated, you need to open the windows to let the steam out otherwise it will steam up the other windows.
