What would happen if I use a power supply with a different wattage than the system is designed for?

You can safely use the 85W adapter on any Mac, it will only draw as much power as it needs. The 45W adapter from the MacBook Air may just about manage to power your MacBook Pro but it won't charge it. It will not damage your Mac, but you may notice the battery may still deplete when using it.

Here is the information, straight from Apple:

You have 2 scenarios as I see it:

1) Using a higher rated power supply than the original unit.

This is absolutely fine. The power supply will never just automatically run at 100 % of it's capability, it will only supply what is requested. If you use a MacBook air that is supplied with a 45 W adapter with an 85 W adapter instead, it will still only draw what it needs, which is likely a whole lot less than the rated capacity of the charger anyway, which is rated to be able to not only keep the computer going at full bore if required, and also charge the battery at the same time.

2) Using a lower rated power supply than the original unit.

Chances are this is fine most of the time but there are more risks. The same rules apply, it will draw the power that it needs, and no more. Assuming you use a MBP on a MBA charger, then light use will likely never draw enough power to max out the charger. If it's in sleep and just charging it will be fine, however the chance is there that if you thrash it it will start requesting more power than the MBA charger can safely provide. There are 2 points to note here:

• Although it isn't stated in obvious terms, the 45 W (or whatever) rating shown on the power supply isn't necessarily accurate, it is in fact the guaranteed minimum continuos rate that it can supply. Changes are that it can supply more, say (depends on the quality of the unit) +10%.

• A high quality unit (such as an official Apple unit, not an eBay Hong Kong special) will have various safety measures to prevent all sorts of things, from overheating, over current, etc etc, and the worse you can except is the unit to fail gracefully with protected systems. Get a cheap unit made from chicken wire and chewing gum though, and you are asking for a fire.

As for charging speeds, the Air likely wont charge any faster at all, but the MBP may charge slower (or, like an iPad running off an iPhone charger, not at all when in use and merely provide enough power to keep the device going).

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:

Example: 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.

If the magsafe plug (1, 2 or whatever) fits then use it. I can assure you there would not be any problem except for the slower charge rate. It wont burn anything. Apple's engineering is topnotch and they wont design the magsafe plug to fit if something will go wrong. You can count on that. Bogdansrc has even attested to it already.

Never mind the wattage (45W, 60W or 85W) the load or the apple unit being charged will vary the voltage and ampere depending on the processing tasks on hand. As Nike says Just do it" Cheers !