The answer to your question is no. An 850 watt power supply does not operate at 850 watts all the time. A power supply will only draw the power at the electrical wall outlet that is needed to operate the computer system.
If you expect to operate using only 425w max, or even up to 500w, I suggest you also consider the Corsair HX 620 which also has excellent efficiency at that level. I don't know where you are planning to buy the HX850 and at what price, but right now newegg is selling the HX 620 for $110 less than the HX850. Note that the listing says the 72 hour sale ends on 9/5. You might calculate how many years of operation it would take to pay back the difference.