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According to this posted answer, a MacBook Pro will throttle because otherwise its power consumption might exceed the amount supplied by the AC adapter.

However, from my experience, my 2012 MacBook Pro has never gone over perhaps 50W of energy usage, as shown with iStat Menus. My power adapter is an 85W MagSafe adaptor. Because of this, it seems like even without throttling the power consumption never would exceed the amount used.

So why would this MacBook throttle if I took out the battery, as suggested by the answer above, even though the power adapter can supply enough power to sustain peak performance?

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The CPU and GPU can draw more power than the AC adapter can provide as the battery serves as a capacitor and reserve. It smoothes out the voltage when a surge is needed. If the system over draws, voltage drops a little and subtle computing errors happen. If the voltage drops too far, the system shuts down entirely and abruptly.

To be reliable and fast, the battery is needed and the system slows down to preserve the correctness of the function. This is a very well tested design, so it’s not an accident that this happens and it’s not for some small reason. This can be measured with oscilloscopes and quantified precisely.

All the measurements you are doing are time averaged. You’ll want to look at millisecond to second level time scales to see this voltage compensation happen.

Apple engineers work from Intel specs on voltage drop curves and tolerances to meet the needs of the power handling on the Intel side as well as manage the battery and charging circuitry and also build in tolerances for aging gear and adapters.

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  • So you're basically saying that, say, a 13-inch MacBook can actually draw more than 61W of power, although it never happens for more than a fleeting moment? – Skeleton Bow Jun 8 '20 at 20:33
  • Yes @SkeletonBow Far more in bursts. Apple even schedules code and manipulates timers to allow the CPU to take micro naps and then hit it hard and then nap again. The energy cost to wake a sleeping processor core is very high, so to the extent work can be queued, the power draw can be far more bursty than you might expect looking at iStat menu. – bmike Jun 8 '20 at 20:41
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    There’s some typos in here that I’ll come back and fix later - I want to dig up the WWDC videos that cover this and maybe tighten up the wording, but the gist should be good enough for a day or three to help people understand why Apple put this in. – bmike Jun 8 '20 at 20:54

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