The only problem with the good PSU calculators like eXtreme Outer Vision's is they can be extremely inaccurate if not used properly. The simpler ones, such as the one on Newegg's website seems to always be inaccurate and always erring on the high side way too much.
In a nutshell, the two biggest offenders in eXtreme Outer Vision's calculator are System Load and Capacitor Aging. The most realistic System Load setting is actually about 70-75%. If one is to be doing Folding or BOINCing or some other kind of heavy load like that for extended periods of time, then 80% or higher is more realistic. This is because this setting is referring to the entire system, including fans, hard drives, memory, the sound card, etc. etc. etc. So 90% means 90% of the entire system, not 90% CPU load or something like that. Notice how they say that 100% means that ALL
of the components in the computer are at 100% load. This again even includes the little things like fans, hard drives, optical drives, memory, the sound card, etc. etc. etc.
For Capacitor Aging, it can depend on the quality of the PSU. For a very low-quality PSU like this one, I recommend going by what is said at the bottom of the page on the calculator:
Electrolytic capacitor aging. When used heavily, or over an extended period of time (1+ years), a power supply will slowly lose some of its initial wattage capacity. We recommend you add 10-20% if you plan to keep your PSU for more than 1 year, or 20-30% for 24/7 usage and 1+ years.
"24/7 usage" means that the PSU is maxed out 24/7. For this PSU, I recommend no more
than 20%. I think 10-15% is about the most realistic choice.
Now, for good quality-made PSUs, Capacitor Aging isn't even a consideration because the capacitors in such PSUs are so good that they're not really affected by aging enough to even set this to 5% (that is, if they had 5% as an option). The only reason I'm saying this is just in case someone reads this who has a good quality-made PSU.