Question regarding gas usage from AC & Electronics

how much gas does A/C use? According the edmunds article http://www.edmunds.com/advice/fueleconomy/articles/106842/article.html they couldn't tell the difference. (I been using A/C lately because it's getting really hot in idaho, + the black sand pearl doesn't help)

Though i only use when I'm cruising, if I'm going up a hill or speeding up to merge into I turn it off. I recall A/C reduces HP?
How much? like a lawn motor 2.5 hp?

Last question is regarding my radio system. It doesn't really affect the alternator right? I only have aftermarket amp 250WATT and a deck. I'm planning on using an 120WATT adapter to charge my laptop on a long trip. That won't affect my gas mileage right? I had a friend whom wine about how it would take power away from the engine...

Theoretically that doesn't make sense to me. An alternator (properly our generate about 80AMPS) and it's charging the battery and when the battery is full, the power is wasted. Take power away from i.e. 120 watt wouldn't really affect charge or gas usage at all. Does this make sense, i just need a confirmation.

 

Your A/C doesn't really take away much power, you probably won't notice the difference at all. Take this into consideration, Amp is Watts / Volts.

I have a 650 watt amp for my sub, 350 watts of light, all this is on when im offroading, so 1000 watts/ 12v is about 83.33 amps drawing from my alternator.

What i usually do is when my truck is on I disconnect my battery and have everything running at once to see if my alternator can support what i have.

 

I heard a mechanic said it takes 10 HP from a 4 cylinder but not noticeable in a V8 cause it already drains a lot of gas. or 1 Km/Liter