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I have a maxed-out 16" MacBook Pro from 2021. I have a 140W USB-C Power Adapter from Apple that I can use with it. I also have a Belkin Thunderbolt 3 Dock that can supply 85W.

I usually use the Mac as a stationary computer and I don't normally use it for prolonged periods of time without a power supply, so the speed of charging is not important to me.

However, I am a software developer, so performance in general, and CPU speed in particular, is very important to me.

The Mac works fine with just the USB-C cable to the dock connected, and it does charge in that state. But I could also have both the dock and the power adapter connected, though it is less convenient to have two cables connected rather than one.

If I use just the dock, with the lower power, it can obviously take longer to charge, but that is not important to me. What I'm interested in: will the performance of the Macbook ever be negatively affected?

I know that the Mac will be slower if I have no power at all connected, but when power is connected, does the wattage of that power ever matter?

If I'm running a really heavy compilation, that uses all cores, can that go slower if I'm using a less powerful power supply?

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  • The battery powers the Mac; the cable charges the battery. Are you sure that the Mac is slower with no cable connected?
    – benwiggy
    Dec 4, 2022 at 14:20
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    Why not run the same compile test with each power supply and compare the results?
    – gatorback
    Dec 4, 2022 at 16:25
  • @benwiggy This is not exactly true, the Mac will be powered directly by the power adapter, bypassing the battery when plugged in. Always going through the battery would put unnecessary wear on it.
    – MJeffryes
    Dec 5, 2022 at 10:05
  • @MJeffryes I'm not convinced by that. The battery is still cycled between 100% and about 95% while plugged in (or c. 80% when charging on hold). If the Mac was powered directly by the cable, why would the Mac throttle when there's no battery?
    – benwiggy
    Dec 5, 2022 at 11:12
  • @benwiggy Under some situations the battery can still be used when plugged in (eg more power is demanded than the power adapter can supply) but it's all under power management. My point is just that it's more complex than "the battery powers the Mac".
    – MJeffryes
    Dec 5, 2022 at 11:16

3 Answers 3

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No, it doesn't affect your performance negatively. Rather it is more likely that having a higher wattage charger could hurt performance (ever so slightly).

The reason it doesn't affect your performance negatively is that if your computer for shorter periods of time uses more than energy than your charger can supply - it simply drains from the battery. Ofcourse if you have sustained loads for a very long period of time (i.e. hours) of extremely high load, then you might simply run out of battery, which you could ofcourse say is a total loss of performance.

However, the difference between 85W and what your MacBook Pro can maximally charge is so small, that in practice this is not something you'll run into - unless you have a really edge case of performance hungry work.

If you connect the 140W adapter to the computer and the battery is rather low in charger, then it can fast charge the battery very quickly. This will generate some heat, and that could negatively affect performance for sudden, very demanding tasks. Ofcourse that only happens in that short period of time where fast charging happens.

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  • Can you explain how the higher wattage could hurt performance?
    – benwiggy
    Dec 4, 2022 at 14:22
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    @benwiggy I was writing that when you asked, it is already in the answer now.
    – jksoegaard
    Dec 4, 2022 at 14:23
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    The reason it doesn't affect your performance negatively is that if your computer for shorter periods of time uses more than energy than your charger can supply - it simply drains from the battery. - don't know about MacBooks, but i've seen Intel laptop CPUs clock down (significantly) in this scenario, using a lower wattage charger on a full battery
    – hanshenrik
    Dec 5, 2022 at 5:56
  • Yes, Dell for example has a system like that on many Windows laptops. The MacBook Pro do not work that way.
    – jksoegaard
    Dec 5, 2022 at 11:06
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TL;DR Yes, it is possible, but unlikely.

In general, the wattage drawn by a CPU is dictated by its power needs at any given time. The power adapter and battery only determine the maximum capacity for power delivery.

Performance may be throttled by factors such as temperature and/or battery health. Intel SpeedStep technology aims to balance this "for optimal performance and power efficiency", and I suspect Apple's M-series chips have a similar feature, akin to Race to Sleep. In any case, if power demand exceeds power supply, performance is likely to be degraded, but for this to happen you must be maxing out the CPU to its absolute limits for an extended period of time.

If you insist on getting an exact answer, you can test it yourself for your own setup. This question might be of relevance to retrieve the current power consumption level, although a hardware power usage monitor will give a more accurate reading. To determine the power drawn from your battery or AC adapter, see this Apple support page for up-to-date instructions.

This answer from bmike offers some more insights into the dynamic between the CPU and its power supply.

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  • The M1 series do not have SpeedStep. Windows laptop manufacturer often use this feature, amongst others, to build their own (or possibly the BIOS manufacturer's own) power management feature on top of the Intel supplied layer. In consumer laptops you often see this scenario where the wattage of the charger is taken into account, and the CPU is throttled or otherwise sped down accordingly. The Apple Silicon Macs do not have something similar to this. Remember that this question is about a laptop, so the wattage of the charger is not directly correlated with "power supply" [...]
    – jksoegaard
    Dec 5, 2022 at 15:02
  • - as that would be the battery. So for Apple Silicon Macs the performance is not degraded.
    – jksoegaard
    Dec 5, 2022 at 15:02
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Not with the things you are doing.

If you use a 45 Watt power supply for example, or an iPad 30 Watt USB C charger instead of the MacBook USB C charger, then your battery will empty, and when your battery is emptied your MacBook will be massively slowed down, say to one GHz, to reduce its power. Then if your tiny little charger manages to get the battery a bit fuller, say ten percent, it will go back to full speed, discharge the battery, it slows down, etc. Over night if you don't use it the battery will charge back to full, and then you can use it for many hours again.

With an 85 Watt charger that's not going to happen. Worst case if you have all cores running permanently - and that's hard to do - your battery would discharge very slowly.

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  • And the computer will become uncomfortably hot.
    – Nelson
    Dec 5, 2022 at 3:27

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