In a recently updated support document, Apple informs iPhone 12 mini buyers that its new MagSafe Charger will be limited to 12W peak power delivery when used with the upcoming device. The charger provides up to 15W for all other iPhone 12 models.
… The actual power delivered to the iPhone will vary depending on the wattage of the power adapter and system conditions. For iPhone 12 mini, the MagSafe Charger delivers up to 12W of peak power delivery.
Apple says the iPhone 12 mini can accomplish the 12W peak with a USB-C Power Delivery power adapter that is at or above 9V/2.03A. However, the support document notes that iPhone 12 power delivery can vary depending on various factors, such as temperature and system activity.
If any Lightning accessories are connected to any iPhone 12 model, the MagSafe Charger will only output 7.5W charging to comply with regulatory standards.
Apple tells users that the MagSafe Charger should be connected to a power source before placing an iPhone on it:
It’s important to plug into a power source before placing your iPhone on the MagSafe Charger. This allows MagSafe to verify it’s safe to deliver maximum power. If you happen to place your iPhone on the MagSafe Charger before plugging into a power source, simply remove your iPhone from the MagSafe Charger, wait three seconds, and then put it back on to resume maximum power delivery.