Apple has outlined in its support document that the iPhone 12 mini’s MagSafe charging will be capped at 12W. Other iPhone 12 models will have MagSafe power delivery maxed at 15W.
The document mentioned how using a USB-C power adapter rated at 9V/2.03A can achieve the 12W delivery. However, the document states there could be other factors such as system activity and temperature that could affect it.
For the rest of the iPhone 12 lineup can achieve charging speeds of up to 15W via a USB-C power adapter at 9V/2.56A or 9V/2.22A. When Lightning accessories are connected the phone accepts only 7.5W max to comply with regulatory standards.
Apple recommends connecting the MagSafe Charger to a power source before putting the iPhone to ensure it delivers maximum power.
MagSafe charging is a new iPhone 12 feature which allows for magnetic and high-efficiency wireless charging. The accessory is available to purchase for $39.