Can you use a 65W apple charger instead of the 45w charger that comes with the Macbook Air? How will this affect the machine.
Yes this is fine (assuming you refer to the 60W Magsafe) and this has been confirmed by Apple to be safe
Although you should always use the proper wattage adapter for your Apple portable, you can use an adapter of a higher wattage without issue.
This will not affect the Mac at all. It will not charge faster. It will not damage your battery. It will not damage your adapter.
Information on power adapters is detailed on Apple's support site: http://support.apple.com/kb/HT2346
To find the right charger for Macbook ,there are three kinds of power rating Macbook power adapter in the market,45 watt, 60 watt, or 85 watt. It should be noted that higher wattage chargers can be used on the lower wattage laptops, but not vice versa.For example,If you have mac laptops that have used all three wattages. you could buy the 85w ones as my back up. Why? Because the 85w will work on any laptop, but the 45 and 60 will not. Further, the cost difference between the 45w, 60w and 85w is virtually nothing so you might as well “go big” and never worry if you have the right adapter for the right laptop.You can also find the Power Rating details on your orignal macbook charger and get the same Power Rating as your origanl macbook charger .
You can use the 85w too, but bear in mind that periodic use of power adapter with more wattage, will decrease your battery life. I can't find the link for that information, but it has been mentioned to me in at least three different Apple Stores by different employees when I lost (twice!) my 45w power adapter and needed to buy a new one; the first stores didn't have stock for the 45w and they strongly advised me against buying the 60w version. I ended up finding a 45w later that day.
Your battery will charge slightly faster and that is not good. There's a reason why each machine has a different battery size and therefore a different power adapter.
Occasional usage of a different model is ok. For example if you're running low and need to keep working or something like that, but don't buy one and use it all the time.