I am surprised that no one has noticed the attempt to power a device who's stated power requirements is "14.5 Volts DC at 3.05 Amperes" (The MacBook Air) with a 20 Volts (!!) DC 85 Watts power adapter.
Yes, I agree, there is nothing wrong with using a higher wattage than the original power adapter for powering the MacBook Air, but it has to be at least very near the originally required voltage of 14.5 Volts DC. 20 volts is just a too much of a jump from the manufacturer's stamped device's voltage requirements. I would accept voltage variations at no more than 2 extra voltages, say, 16 volts DC at 85Watts or whatever wattage, but never would I dare power any equipment that says that it needs 14.5 Volts with a 20.x power adapter. That's, in my opinion, is asking for trouble, and I am surprised that many users here say that they have been doing it for months and it's working just fine. It's a gamble and I am glad it's paying off for these users, but me, as a computer engineer, I would never do such a thing.
I have with me a MacBook Air that one of my computer repair customers just dropped off at my computer repair shop, stating that it doesn't turn on. Upon examining it, it appears that its AC Adapter is at fault. Problem is that Apple decided to ship the A1369 MacBook Air whose power requirements are: 14.5 Volts DC at 3.1 Amps with a 14.5 Volts DC 45 Watts AC adapter. 14.5 Volts times 3.1 Amps equals 44.95 Watts, yes, the Maximum energy that this particular product will ever draw from the power adapter is 44.95 watts. Apple merely gave this pesky 45 watts power adapter a 0.05 watts head room when the customer is charging the battery and the customer is maxing out the CPU and GPU which I assume would be the conditions for the MacBook to consume its maximum rated energy at any given time and on these times the adapter is being taxed at 100% of its capacity it can deliver, no wonder it burned out. Apple should have shipped this MacBook Air with at least a 60 watts adapter so that it can have a head room and not be working 100% in any given time so that it can actually last. Well, guess what, I will be providing this customer with a 60 watts adapter since that would be the most responsible thing to do, but I would never provide a 20 Volts power adapter to power a 14.5 volts device.
So, in conclusion, be careful with attempting to over volt any given device with an input VOLTAGE greater than what it needs. Remember, you can power the 14.5v MacBook Air with a 14.5 volts AC adapter of any wattage of your choice greater than the baseline wattage of 45 watts:
It's perfectly alright to:
* Use a 14.5 volts DC of any wattage that happens to be a minimum baseline wattage output of 45 watts to power the MacBook Air. The adapter can say that it can power up to 1,000 watts at a rate of 14.5 volts DC and the MacBook Air will be alright, perfectly fine. The Mac will only draw the first 45 watts off the adapter, not needing to use the rest that the adapter can supply.
* Use, let's say, a 16.5 volts DC of any wattage. I wouldn't go too high on the wattage here as I would be over volting the input by a factor of 2 volts, and it is almost certain that the MacBook, along with most notebook computer, have a circuitry in place (voltage stabilizer) to keep the "working" voltage stable at its best operating voltage, despite irregularities of the input voltage which is probably trues given that many of you are being successful at powering your 14.5 volts MacBook with a 20 volts adapter without burning out some components, but the voltage stabilizer can only work for a certain range, and the higher you go in terms of voltage, the more risk you go in terms of burning out the voltage stabilizer, thus causing your Mac not to power on again under any adapter.
It's not alright to:
* Power your 14.5 volts MacBook Air with a 20.x power adapter of any wattage rating, even if it does work at the moment, because the voltage stabilizer has to work extra in order to down convert the voltage to 14.5 volts so that the rest of your Mac doesn't smoke up, and most likely this is causing the voltage stabilizer/regulator to heat up beyond its rated specs, which once again, translates to a shorter lifespan of the MacBook Air.