The best speeds come from charging using the 20W power adapter without a case on the ‌iPhone‌ to better let heat dissipate. Zollo's testing also revealed that Apple is using aggressive temperature control, so when the iPhone gets warm, the charging power tends to stay below 10W. Third-party companies will need to come out with new chargers that use the particular power profile that Apple is using to provide the optimum amount of power before a third-party charger will be able to provide the full 15W with the ‌MagSafe‌ Charger. Anker 30W PowerPort Atom PD 1 = 7.5W to 10Wįor maximum charging speeds with the ‌MagSafe‌ Charger and an ‌iPhone 12‌ or 12 Pro, Apple's 20W power adapter is required, and older power adapter options won't work as well.Apple's 96W MacBook Pro Power Adapter - 10W.Below are the results from Zollo's tests: The older 18W power adapter from Apple that was replaced by the 20W version was able to charge the ‌iPhone 12‌ Pro using the ‌MagSafe‌ Charger at up to 13W, but the 96W Power Adapter and third-party power adapters that provide more than 20W were not able to exceed 10W when used with the ‌MagSafe‌ Charger. Paired with the 20W power adapter that Apple offers, the ‌MagSafe‌ Charger successfully hit 15W, but no other chargers that he tested provided the same speeds. YouTuber Aaron Zollo of Zollotech tested several first and third-party power adapter options with the ‌iPhone 12‌ Pro and a ‌MagSafe‌ charger using a meter to measure actual power output. Apple does sell a new 20W power adapter alongside the ‌MagSafe‌ Charger, and as it turns out, that seems to be one of the the only charging options able to provide a full 15W of power to the new ‌MagSafe‌ charger at this time. So at a minimum, ~50W to supply 45W (which is only enough for a MBAir and not enough for a Pro).Apple does not provide a power adapter with the $39 ‌MagSafe‌ charger, requiring users to supply their own USB-C compatible option. There does not seem to be a USB-C PD → MagSafe2 option, possibly because it would require a charger that supports enough wattage to both power the MacBook (45W/65W/80W) and the dongle itself.

Any USB-C → MagSafe2 dongle would essentially ensure the wattage via USB is available and emulate the correct MagSafe2 charger ID.

It won't draw more current than the charger will allow because it knows which charger is plugged in. The MacBook knows what the max wattage for the charger is by getting the id of the charger from the MagSafe2 plug itself. There are only 3 wattages for MagSafe2 chargers and they have well known voltage / currents.

As people have noted, there are plenty of MagSafe2 → USB-C PD options. Electrically, it is possible to go from either direction, it just requires a computer (microprocessor, etc) in between to do the USB-C PD negotiation.

Magsafe usb c adapter