If it is a third-party charger with C port for the iPhone12 fast charge, may wish to refer to my test results with 30W charger, and 120W charger is the same:
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The continuous charging power of iPhone12 can reach more than 19W under the off-screen state, and less than 20W, and I have reached the maximum input power of 25W under the on-screen state.
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The whole screen charging, starting from 20%, the power is maintained at 19w for ten minutes and then dropped to about 15W.