18 w。
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The iPhoneX Fast Charge uses the USB PD charging protocol, and Apple is one of the members of the charging protocol alliance.
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At this stage, Apple's official chargers that can be used by the iPhoneX to support fast charging have five USB-C interfaces: Apple 18W, 29W, 30W, 61W and 87W.
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The USB PD protocol is a protocol handshake between the charging head and the mobile phone (the charged device).
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That is, when you plug an Apple original 87W (Apple Macbook Pro's original USB-C charger) into an iPhoneX for charging.