Simple way is to look at the power-adaptor... It provides the power, and thus, indicates the power-rating. (Assume it runs at 90% consumption, when mining. 100% if you kill the charger.)
EG, if the output says 14v @ 20A then it is 14vx20A=280W... Then add 15% to that wattage, as most wall-supplies are 15% inefficient. So, at the wall you will see about 260-300W consumed, to produce 200-280W of output power. (Remember, you battery is charging also, which consumes MORE when it is in a charge-state, and less when resting.)
You can provide more "usable power" by blacking-out your screen, turning off the brightness/backlight, shutting-down "indexing services", turning off "defrag services", disabling blue-tooth, cameras, sound, switching to lower resolution or the output-port, etc... Leaves you with more available power for hashing... but if you are not making over 300MHs, then you are paying to hash, as that is roughly about equal to the power you are using to hash. That is ONLY if you cash-in those coins, which takes-away more value from fees and your time to cash them in... all $3.00 a month.
But I am only using 65W. Surely I don't need 300MHs to break even on 65W?
