The MagSafe charger itself does not have a power supply. It needs to be used with Apple’s 20W USB-C charger. If you have the 18W power supply that comes with the iPhone 11 Pro, then of course you don’t need to buy it again, but if you don’t have one, If you want to use MagSafe again, you must buy the official Apple charger instead of other USB-C chargers!
In a test conducted by YouTube channel Zollotech, it was found that the MagSafe charger can get the highest charging performance on Apple's official 20W power supply or the 18W power supply of iPhone 12 Pro.
According to the power test table, using Apple’s official 20W power supply, charging can reach 15W, while using Apple iPhone 11 Pro’s 18W power supply, charging can only be 13W. How about using the MacBook Pro USB-C 96W power supply? The answer is that MagSafe charger is limited to 10W. If you use the power supply of Google Plxel, Samsung Note 20 Ultra, the charging is only 9W.
In addition, if you use MagSafe charger to charge iPhone 11 Pro Max and iPhone 8 wirelessly, MagSafe can only reach 5W, but other Qi chargers can reach 7.5W.