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One of the cable tabs on my MagSafe charger for my MacBook came undone, so I took this as a sign to retire the charger to home use and to purchase another for travel. When I went to my local Best Buy, they had two chargers for sale, 60W and 85W, that looked the same size, sold for the same price, etc. The only difference was that the 85W supported the MacBook Pro, while the 60W only supported the MacBook.

Even though I am a MacBook owner, I figured I might, sometime in the future, purchase a MacBook Pro and I went with the 85W charger. When I opened it up I found it was slightly larger than the 60W charger it was replacing, but for all intents and purposes there's no real difference between the two. So why does Apple sell a 60W replacement (or even manufacture the 60W at all)? Are there enough people that want to save 25W?

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+1 Exactly what I was wondering. –  Ergwun Feb 18 '11 at 3:45
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4 Answers 4

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Most switching power supplies are most efficient when operating at 50% capacity. And they're definitely pretty inefficient at less than 20% capacity.

Since the MacBook draws less power than a MacBook Pro, I would suspect that an 85w power supply would work just fine, but the larger power supply for the MBP probably would draw more power from the wall socket than is necessary when connected to a MB because it is operating in a suboptimal capacity range.

Here's an example: Take for example the 45w power supply for the MacBook Air. Sure you could hook up the 85w power supply, but since the MBA might only draw 20 watts, the power supply is operating at less than 25% capacity. At this capacity, the power supply may only be 60% efficient (an educated guess), that means the power supply has to draw 33 watts of AC power just to supply the laptop with 20 watts of DC power.

Long story, short: It's a power conversion efficiency thing. I'd say buy the one that is designed for your laptop so that you save energy.

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I suspected this, which is one of the reasons why I like that I'm leaving my 60W at home and using my new 85W on the road. –  Kyle Cronin Jul 27 '09 at 20:38
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@Ranger: To clarify, the article shows a power supply that still operates at almost peak efficiency at slightly less than 20% load, whereas you claimed that power supplies are "pretty inefficient at less than 20% capacity." You can speculate about the efficiency of any given power supply, but without actually measuring it, you're just shooting in the dark--especially since the load will vary depending on the user's habits. Given the same two laptop power supplies, one could be more efficient for my usage pattern while the other is more efficient for yours. –  rob Mar 2 '10 at 2:51
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@Ranger: Also, I don't think the omission of the 0W-100W range was an attempt to "hide" anything. The author simply did not expect his target audience to be interested in power draws of less than 100W. –  rob Mar 2 '10 at 2:56
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@Pretzel: My original point was twofold: first, without measuring the efficiency of both chargers you can't just claim one is more efficient than the other, as you did in your answer. Second, each one will have different efficiencies under varying loads, and these loads are directly related to a given person's typical usage pattern. I guess I was hoping that, rather than speculating on which charger is more efficient (the original question), you would update your answer to acknowledge that there is no way to know which PSU is more efficient without actually measuring both under varying loads. –  rob Mar 5 '10 at 19:36
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@Ranger: Your last couple comments smack of fanboyism, which I suppose explains why you seem to think that your suppositions count as hard proof that the power supply that ships with a laptop is the ideal power supply in all respects (cost, weight, aesthetics, efficiency, reliability, etc.). The real world, unfortunately, is not so perfect, and compromises were likely made which adversely affected one or more of the power supply's characteristics under various loads. –  rob Mar 10 '10 at 4:29
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http://discussions.apple.com/thread.jspa?messageID=6301678

Sounds like it does work just fine. You cannot use the smaller charger with MBP of course. This is also the case with the MBA which uses a 45watt version.

In addition to what rangerpretzel said, which is the main technical reason:

But also, size matters to some, MBA users want portability. the 45 watt version is much smaller and the 60 is smaller then the 85. Some people want the smallest one compatible with their computer. Also, the original 85 watt magsafe power supply used to be even larger then it is today and was a concern at the time of release. Apple eventualy re-engineered it to the size it is now.

Cost wise, it might make more sence to just make one power supply, but apple does offer its users a "few" options. haha.

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The MBA air is a different adaptor because of the angle the port is at which is a result of the case design. A MB or MBP adaptor just wouldn't fit and a MBA. Even if it did fit would be underpowered for a MB and shockingly, probably dangerously, so for a MBP. –  JonHopkins Jul 27 '09 at 20:22
    
Oh, you kinda mis read what i meant. I just mean you "could" possibly use the larger power supplies with the smaller models, not the other way. –  Troggy Jul 27 '09 at 20:25
    
You never want to use an underpowered power supply, major fire danger there. –  Troggy Jul 27 '09 at 20:27
    
And yes, you are correct about the angled connector. If it was not angled, you could potentialy use a larger power supply, but its not recommended, needed, or efficient. –  Troggy Jul 27 '09 at 20:29
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You definitely don't need to worry about your laptop being fried by the higher-capacity power adapter, because the laptop will only draw what it needs.

My 6-year-old laptop came with a 65W power supply. When I bought it, I could have gotten it with a 90W power supply, but I opted for the smaller power supply. The docking station that I purchased later came with a 90W power supply which can also plug directly into the laptop. I've noticed that the 90W supply charges the laptop faster, but I prefer it mainly because it has a much longer power cord.

The efficiency figures Ranger has hypothesized are not necessarily accurate; it depends on the design of the specific power supply and the optimal load range. A given power supply may be more efficient under your own typical usage scenario, but the only way to find out for certain is to test each power supply's power draw yourself under different loads. For example:

  • Plugged into the wall, but not into a laptop
  • Laptop fully charged and turned on (idle)
  • Laptop fully charged and maxing out power draw (e.g., transcoding video while burning a CD)
  • Laptop charging but turned off
  • Laptop charging and turned on (idle)
  • Laptop charging and maxing out power draw (e.g., transcoding video while burning a CD)

You may find that the higher-wattage power supply is actually significantly more efficient under a high load than the lower-wattage power supply, because it is not near its maximum capacity. In fact, you may find that the higher-wattage power supply is more efficient across the board, and the lower-wattage power supply only beats the bigger supply under a narrow range of loads. Or perhaps the smaller supply is never more efficient, and its only advantage is its smaller physical size.

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I like where your head is at. I have a 60W MB adapter which has all but melted under the loads I frequently subject it to. Such as charging, compiling, and ripping a DVD (DVD read + video transcode) all at once. –  sholsinger Aug 25 '11 at 18:21
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Most of the anecdotal evidence is that there are no problems, and this extends to a member of Apple Store staff who say they use MBP charger to charge all laptops and if you read the small print they are suitable for both MB and MBP.

I did find one person who said that their batteries "burnt out" while using the 85w charger with a MacBook but this seemed so far from the normal tale (that everything was fine, of which there are hundreds of accounts) that it may have been down to something else.

But it doesn't answer the question why do apple do it. It would be simpler (certainly) and cheaper (probably - just down to bulk buying) to standardise. The fact they haven't makes me think that there may be a reason, however it may be historic. For instance it may be that the new MacBooks may run fine on either charger while the old ones don't. To avoid having to specify model and year and risk selling someone a charger which will melt their old laptop, they may have just decided to stick with MacBook charger and a MacBook pro charger and avoid confusion.

The 60w adaptor incidentally is underpowered for the MBP - battery charging takes longer and it will get very very hot in extended use. It works but really not advised.

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I'm also curious what power supply the 13-inch MBP uses. I suspect it's still shipping with the 60W adaptor, which can only serve to further confuse the issue. –  Kyle Cronin Jul 27 '09 at 20:54
    
If my hypothesis that this is a historical / standards thing is right then Apple would want to ship it with the 85w supply (a) because this is the MBP supply and (b) because at some point in the future when they're happy that all MBs can use the 60w supply they'll move to that as standard and this would be an early step. –  JonHopkins Jul 28 '09 at 8:03
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