If you ask Apple Support if the 18-watt power adapter that came with the iPhone 11 Pro, iPhone 11 Pro Max, and four different iPad models will work with the MagSafe Charger and your new iPhone 12 or 12 Pro, the answer is "yes." But that's not the whole story.
People have been posting their interactions with Apple Support about the issue, and the answer is always, yes, the 18-watt power adapters work with the new MagSafe Charger. I even asked about ten different people in Apple Support chats and got the same response each time. Big-name Apple enthusiasts like ZolloTech even say that the 18-watt power adapter will work just fine.
But Apple's website has this to say about fast charging with the MagSafe Charger:
Note: For fast charging, iPhone 12 models require a power adapter with a minimum power output of 20 watts, such as the Apple 20W USB power adapter. If you use a third-party power adapter, it should follow these recommended specifications:
- Frequency: 50 to 60 Hz, single phase
- Line Voltage: 100 to 240 VAC
- Output Voltage/Current: 9 VDC/2.2 A
- Minimum Power Output: 20 W
- Output Port: USB-C
In another support document, Apple even says, "You need a 20W or higher power adapter to fast charge your iPhone 12," referring to wired Lightning to USB-C connections instead of wireless charging, but I find that hard to believe.
So will your 18-watt power adapter really fast-charge with the MagSafe Charger? Yes and no. Unlike wired connections, there is some drop-off that happens between a wireless charger and the phone.
If you have an 18-watt power adapter, there are only 3 watts to spare if you want to hit that 15-watt sweet spot. Since wireless chargers are never that efficient, even with Apple's new MagSafe technology that aligns everything perfectly, that 18-watt power adapter will drop you down to a charging speed of 12.5 watts. That 12.5 watts was confirmed by multiple Apple representatives we talked to, but the exact amps and volts measured may vary.
Apple needs that full 5 watts to account for the drop-off rate, which is why 15-watt fast charging will work with a 20-watt power adapter that has the minimum above specs. Now, 12.5 watts is still technically fast charging, especially considering that's 5 watts faster than the older iPhones that top off at 7.5-watt wireless charging. So you won't be in a worse situation than before if you didn't buy the 20-watt version with your new iPhone 12 or 12 Pro and MagSafe Charger.
While at first, it seems like a money grab to get you to buy Apple's 20-watt power adapter instead of the 18-watt one (which it stopped selling), it's to make sure you get the advertised 15-watt fast charging speeds. Instead of getting knee-deep into all of the technical details, Apple just stopped selling the one that won't get the 15-watt speeds and is pointing you towards one that will. But tech support will still say the older 18-watt charger will still fast charge with the MagSafe Charger, and they're not wrong — they just not giving you the full story.
TL;DR: Stick with your 18-watt power adapter for fast charging at 12.5 watts, or upgrade to a new 20-watt power adapter for the full 15 watts.
Updates published: (1) Added that Apple confirmed the lower speed on background; (2) Added another support document discussing fast charging with 20-watt adapters.
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