Q: Is the 135W Smart AC Adapter compatible with the HP Compaq 6910p? A spreadsheet I got from some co-workers said it was compatible, and when I plug it in it seems to work with not errors being noted on the 6910p. I haven't been able to verify if it is in deed compatible.
Also I see that the 90W Smart AC Adapter is compatible with the HP Compaq 8710w. When I plug it in it warns me to use the larger power supply: "For full performance, connect a higher capacity AC adapter." What limitations will I have with the lower supply adapter on the 8710w?
A: I agree - go for the 135W units as spares.
On your second question, the power rating of the adapter in Watts (power = volts x current) is the maximum POWER it CAN deliver - it will only deliver what the laptop needs within that limit. The adapter is basically designed as a constant VOLTAGE source, and thus is only capable of providing a limited flow of CURRENT.
In reality, as the load increases (in effect has less resistance so the current increases (Current = Volts / Resistance) the voltage output from the adapter will start to reduce in an effort to ensure that its maximum power is not exceeded.
The volts start off the same for either adapter - but as more power is demanded by the laptop (eg it wants the processor running at full speed and all the disk drives running - and the batteries need charging too), the laptop will try to draw more current (amps) and hence the total power it draws will increase. Clearly, the 90 Watt unit will run out of steam before the 135 Watt unit !!
This link gives a short but very clear and succ