There's only so much current that you can draw from a 15A circuit - and that translates into 1800W. I'm not sure about the idle draw but it's likely to be small (most folks use the trigger to shut them down when the system's off). Actual consumption values are going to depend too much on other variables (speaker efficiency, volume and acoustics of listening room, source material, and volume). Actual electrical consumption would probably be in the neighborhood of 3A or 4A in many cases (a conservative guess) even at higher volume levels, and that equates to around 400W or 500W total. That's just a guess, though - you could see cases where you don't draw more than an amp or two, or a system with inefficient speakers in a large room with an aggressive surround mix at reference level could probably sustain 7A or 8A. I've got some numbers in my design notebooks at work from when I tried to estimate the heat load of a multichannel amplifier, but as you can tell there are a ton of variables involved and there's no research data that I've found that provides any support for my estimates.