Couldn't find it in Onkyo's promo literature, but it's buried in their online manual on p. 65 at the top of the right side.
It's rated to draw 5.9 amps, 0.1 amps at idle.
Here's a link to Onkyo's online manuals. The HT-S790 is pretty close to the bottom of the list. It requires Adobe Acrobat reader to download the manual. The online manuals for my TX-SR600 and the receiver in my HT-S780 system are an exact copy of the ones that came with the actual product, not abreviated in any way. Good info awaits therein.
If you're wondering about circuit capacity, I run my 780 system, which has a similar receiver, on a standard 20 amp circuit, which also powers other items such as lights and wall plugs, without ever tripping the circuit breaker, and I push the piece pretty hard when listening to hi-rez music. The only thing I'd recommend is that you make sure you give the receiver plenty of ventilation--it does tend to get hot when driven hard (and warm even when not driven hard--it's an "Oknyo thing", I guess).
Retired twice, back at work again...but still attempting to age disgracefully!