Nope. Power supplies are rated on output. At best efficiency would be 90% at high end of the curve. So 900W/0.9 = 810W.
No the best efficiency is near the middle load. Get a bigger power supply than you need. It will save money on energy, run cooler and quieter, and probably last longer. Also if you can run on 240, that gives another few percent usually.
Typo. I was saying at most the PSU will be 90% efficient. Still the efficiency curve on a modern "80-Plus" PSU is very very flat, almost a horizontal line. The difference between 50% load and 80% load is usually 1%. It isn't going to run significantly cooler, last significantly longer, or save any meaningful amount of power. Key word meaningful be sure to compare any savings to the cost of oversizing the PSU. It isn't the 1980s anymore. PSU have gotten a lot better in the past 3 decades.