Let's not forget the 5volt rail in that wattage equation.
Also, the power supply is a 120 watt power supply, but what is it rated at on just the 12 volt rail (where all the spin up power comes from)?
Then... While the power supply might be rated at 120 Watts (power supply ratings are always a bit speculative), but that is supposed to be continuous, not peak. What is the power supply rated at for peak load, on th 12 volt rail?
I swear, I am not trying to be a smart ares here, promose. My point is that determining the power we can use is not as simple as we have put it here. To do it that way, we would need to know the actual peak load rating/capability of the power supply on the 12 volt rail. Our best bet is to go by examples of what has worked and what has not worked for others, -OR- find out these numbers (actual peak rating, and actual spin up loads).