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My late 2011 17" MBP was delivered with an 85W MagSafe adapter.

Under heavy stress (either CPU and/or GPU) the MBP has to draw additional power from the battery in order to keep everything running. Though I can't imagine this being particularly healthy for the battery (as it keeps starting to charge a tiny bit every chance it gets, and stops again when power consumption goes up), this seems to be 'expected behavior by design', if I'm to believe the various discussions on the internet.

What strikes me as odd though, is that the 85W adapter isn't actually delivering a full 85W when the MBP starts pulling additional juice from the battery. It stalls at about 77W, according to the DC In sensor. This doesn't seem right to me.. why leech power off the battery if the preferred supply has energy to spare?

So, in essence, my question boils down to this:

  • Is anyone else seeing similar behavior?
  • Does anyone measure higher throughput from the 85W adapter than 77W? (charging / leeching both maxes out at this rate more or less)
  • Would the adapter be actually delivering 85W, and is the DC In sensor just plain wrong?
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When measuring the usage of power adapters, it is important to keep in mind the efficiency of the PSU when making calculations.

For PC builders, it's important to ensure that you choose a suitable PSU for the components you intend to use, to ensure it's big enough etc. As such, the rated capacity is shown as what the PSU is capable of delivering, rather than what it uses. So, for example, a 600w rated PC power supply for a desktop computer that is rated at 80% (considered the minimum in order to get the "80 plus" certification) will actually use up 750w at the wall. 750W * 80% = 600W.

For laptops, the maths is often considered the other way around. As you are unlikely to be putting extra drives, graphics cards, PCI cards etc in your laptop, its internal power delivery capability should be of little interest to your average user, but the external wall-draw might be. Given that a laptop power adapter will likely be more efficient (as it is smaller, uses less power, will therefore run cooler, and require less active cooling in terms of fans etc etc), then it wouldn't surprise me to see it up around the 90% efficiency mark. Add to that the fact that your average laptop user should only really need to know the wattage rating when using the laptop somewhere unusual, say a plane, or some other mobile/portable power source, it may well be more useful to know the exterior power rating rather than the internal delivery rating. Therefore, 85W at the wall * 90% efficiency is 76.5W in the laptop, and likely explains the discrepancy.

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  • That actually makes a lot of sense! 80/90% efficiency seems about right for such a device. What I don't understand though, is that the adapter has an output of 18.5V at 4.6A (according to the text on the brick at least). Following P=V*I, that calculates to 85.1W, which would mean that 85W is in fact the output rating instead of input consumption. Or are the figures on the brick merely a theoretical maximum?
    – Remco Overdijk
    Jan 31, 2012 at 21:44
  • I'm not 100% sure what the figures on the block represent, one thing is for sure any stated ratings are likely to be a) underrated for safety, and b) therefore not wholly representative of the actual capabilities. My 45w Magsafe shows a 14.5v * 3.1a = 44.95 output, but this is likely to refer to the capabilities of what you need to plug it into rather than what it will pass through.
    – stuffe
    Jan 31, 2012 at 21:57
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    Generally speaking you will be able to pull through far more than the rating unless the computer has the relevant safety features to prevent this. I recently replaced a friends 70w psu for a laptop that needed 120.. It worked (although the machine would occasionally crash when playing games), but the PSU was stupid hot, and even bubbling the stickers! Fire waiting to happen. It looks as though the MacBooks have something to stop this, which is a good thing.
    – stuffe
    Jan 31, 2012 at 22:02
  • Answers this good are likely to sway voters in the upcoming moderator election. Either that or get you elected as staff electrical engineer explainer.
    – bmike ♦
    Jan 31, 2012 at 22:18
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    Just a note, I have an 85W adapter plugged into a Kill-A-Watt charging my MBP retina and when charging the battery, maxing the screen brightness and doing things to max the CPU, I see a max of about 94-95W of AC input to the power adapter. At this time, I see about 79-84W from the "DC In V" * "DC In A" that iStat shows. It seems "85W" on the label means "85W of output"
    – kbyrd
    Jan 17, 2016 at 17:54

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