Quitting XBMC returns back to the AppleTV interface. Not sure if it goes into standby from there. You can buy something called a "Kill-A-Watt" for like 20-30 bucks that you can use around your home to check electricity consumption on all your devices.
Another example of how AMAZING the AppleTV2 really is... Powerful video player... 2 watts electricity. You can leave it on for 500 hours before it even uses 1kwh... How amazing is that???!!!