I ran across this tweet and decided share some old knowledge about the subject.
@codinghorror Overheard in computer store "will the 65W power supply instead of 95W slow down the laptop or decrease the screen resolution"chx (@chx) January 11, 2014
If your device ships with a 95W power supply and you hook it up to a 65W power supply you will probably fry the device. When a device is not getting enough wattage it struggles to keep up; damaging the device. If however, you give a laptop requiring 65W a 95W power supply, you are probably fine. The device will simpy take what it needs and ignore the rest. This applies not only to laptops, but to all devices requiring a DC converter (what we call a power supply, usally refers to an AC to DC converter). My advice is to buy your replacement or backup power supply from the manufacturer, this way you know you are getting exactly what you need. This however is not always a feasible option.
How do I choose the right power supply when a manufacturer replacement is not a feasible option? There are three things you need when choosing a power supply. The physical demensions of the tip, the polarity of the tip and the required wattage of the device. When buying a third party power supply for a laptop, you will typically get an AC power cord, the DC converter and package of tips. When buying for smaller devices, the AC power cord and DC covnerter are often times a single unit and the tip may be a seperate item. Some retailers, such will have a selection of DC coverters and tips in which case you can pair the items yourself. Many vendors will have look up guide that will give you the part numbers your device will need. Sometimes you will need to figure that out on your own.
The physical tip is probably the easiest to identifiy. I always reccomend taking the device and the tip with you, some retailers will allow you to plug the tip into your device prior to make the purchase. If you do not have a part number this is the easiest way to confirm a match.
The polarity is either positive or negative, the wrong polarity will fry the device.
The wattage is typically listed on the device. If not you will probably see Volage and Amperage. Multiple the two, this will give you the required wattage
19.5V * 3.33A = 64.935 (always, always, always round up) in this case you will want a power supply of at least 65W.