You can use a UPS or a Killawatt to measure how much total power the receiver is drawing from the wall. I actually just did this a few days ago myself. Watts per channel will never be constant and most of the power will be going to the front speakers.If you are using all 13 powered channels on this receiver how many watts/channel will you end up with?
To give you a general idea here is my system. I have a 7.1.4 setup with speakers that are rated at 150w max on paper. I have an external amp that is powering the 3 front speakers and the Denon is handling the other 8 speakers. The subwoofer has its own power of course.
I have a 1500VA UPS that is connected to only the Denon, the Amp, and an Oppo Bluray player. So the UPS is reporting how much power is being used by all 11 speakers in total plus a few other things.
Most of the time the UPS was reporting under 200w even in action scenes. During heavy moments it would go above 200 and spike above 300. The only time I saw it go higher was the 747 demo when it spiked above 500. This is at Audyssey calibrated reference volume.
Obviously this is not an exact measurement but it gives you a rough idea. TLDR: Its not as much as you'd think.