What would happen if I use a power supply with a different wattage than the system is designed for? My girlfriend has a Macbook Air.  The power supply is smaller and lighter than the Pro.  It has an output of 14.85V and 3.05Amps.  My Pro Retina has a power supply that is 20V and 4.25 Amps.  

Both supplies have the same Magsafe adapter, so the power supplies seem interchangeable at a physical level, but what would happen if one of us plugs in using each other's adapter? 


Could damage result from under-powering my MacBook Pro? (like a brownout)?
Could damage result from over-powering her Air? (like a power surge)?
Will the Macbook Air charge faster with a Pro adapter?
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: