I have several laptops of various brands. Each has an AC adapter that's designed for the specific computer it came with. I also know there are universal adapters can adjust their voltages automatically to be compatible with multiple models.
I'm able to use AC adapters from different companies that have the same voltage regardless of their amperage rating (assuming they're physically compatible), yet if I use two adapters with different voltages and the same amperage, it either fails completely if it's too low or, from my experience, if it's too high something inside the laptop sizzles and pops and the computer's now a brick.
I know that you can use a power supply with a higher amperage rating than you need because electronics don't have to draw 100% of this to function. They need at least their rated amperage, but any extra won't hurt.
A lot of people shop for power supplies based on wattage. That seems to be the golden cow of computer power supplies. When looking at laptop power supplies, however, it isn't very helpful.
The only laptop adapter I have that even lists wattage says:
12V --- 4.0A
- MAX 49W -
I understand that there's a formula for determining watts from volts and amps, yet the max wattage on this power brick is very close to watts times amps. Many laptops have desktop grade parts, yet they come with these same power supplies.
Clarification: How can a small AC Adapter give a laptop enough juice to run a desktop grade GPU when a desktop PSU is much larger and requires a fan and heatsinks?