The peak fast-charging power of the iPhone 8 can reach about 18W, and then the charging power stabilizes at about 15W. According to official data, the iPhone 8 can be charged up to 50% in 30 minutes with the fast charge.
The iPhone 8 fast charge requires a fast charger that supports the USB-PD protocol and a USB-C to Lightning charging cable.
The included 12W charger can also charge the iPhone 8 fast.