The 60w adaptor incidentally is underpowered for the MBP - battery charging takes longer and it will get very very hot in extended use.
#APPLE MAC CHARGER REPLACEMENT PRO#
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. For instance it may be that the new MacBooks may run fine on either charger while the old ones don't. The fact they haven't makes me think that there may be a reason, however it may be historic. It would be simpler (certainly) and cheaper (probably - just down to bulk buying) to standardise. 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.īut it doesn't answer the question why do apple do it. 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. Or perhaps the smaller supply is never more efficient, and its only advantage is its smaller physical size. 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. 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. Laptop charging and maxing out power draw (e.g., transcoding video while burning a CD).Laptop fully charged and maxing out power draw (e.g., transcoding video while burning a CD).Laptop fully charged and turned on (idle).Plugged into the wall, but not into a laptop.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. 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. 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 docking station that I purchased later came with a 90W power supply which can also plug directly into the laptop. When I bought it, I could have gotten it with a 90W power supply, but I opted for the smaller power supply. My 6-year-old laptop came with a 65W power supply. 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. I'd say buy the one that is designed for your laptop so that you save energy. Long story, short: It's a power conversion efficiency thing. 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. 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. Here's an example: Take for example the 45w power supply for the MacBook Air. 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. And they're definitely pretty inefficient at less than 20% capacity.
Most switching power supplies are most efficient when operating at 50% capacity.