This question already has an answer here:
Two questions on points not covered in either of these two threads about AC adapters:
1) Watts aren't mentioned. Is it okay to replace a 65w AC adapter for a laptop computer with 90 or 120w?
2) Voltage: It says an adapter must not have higher or lower voltage than a device's specification but it doesn't say how much higher or lower, e.g. whether a difference of 1 volt out of 20 is a rounding error, or physically meaningful. Specifically, is it okay to replace a 20v adapter with a 19v one, for a laptop computer?