I've been using an old Apple 5 W charger to charge my iPhone 12. Recently I read that it is dangerous to use a charger with too low a wattage, and I see advertisements for 20 W chargers for recent iPhones. Can someone explain what is the best wattage charger to use and why?
There is nothing wrong with using the 5watt power adapter. Apple encourages it. See the following from Buy iPhone 12 - Apple
We encourage you to re‑use your current USB‑A to Lightning cables, power adapters and headphones, which are compatible with this iPhone model. But if you need any new Apple power adapters or headphones, they are available for purchase.
The advantage of a 20 watt power adapter is that it can be used for fast charging. See information in: Fast charge your iPhone - Apple Support
There is nothing wrong with using the 5watt power adapter. Apple encourages it. See the following from Buy iPhone 12 - Apple
We encourage you to re‑use your current USB‑A to Lightning cables, power adapters and headphones, which are compatible with this iPhone model. But if you need any new Apple power adapters or headphones, they are available for purchase.
The advantage of a 20 watt power adapter is that it can be used for fast charging. See information in: Fast charge your iPhone - Apple Support
What wattage charger should be used for the iPhone 12?
