What does old have to do with it? Yes, it has been around for years but it is still being regularly updated with new devices by a regular staff of researchers. You make it seem like it hasn't been updated in years. That simply is NOT true.
Old in this case simply means well established - and for good reason. It is good. If you know of a new device not yet in their database, a quick note to their contact us page typically gets it added within a few days.
The problem with the manual approach is what I already illustrated above. You "guessed", for example, 10W for SSDs yet some draw 20W. This means you have to carefully go out to each manufacturer's webpage for the product, and HOPE power consumption is listed. Other wise, you are just guessing.
Nah! Transients are not a factor. Not sure where you are getting that. It seems you have been misinformed. PSU size depends on power demands, not transients. And power requirements are based on "peak" demand, not average so again, not sure where you are getting "transients". They are not a factor you need to compensate for with an even bigger buffer.
No they aren't. Again, it seems you have been misinformed. For one, it varies by supply. And for another, most are most efficient between 50% and 60% load. See
here (scroll down to 80 PLUS logos) and hover over each logo. You will see a bubble appear showing the peak efficient is at 50% load.
But the point is moot because, as I already noted, if you get an 80 PLUS certified supply (and you should), the efficiency is relatively flat across the full range. "Gold" for example, is an excellent 87 - 90% efficient from 20% load up to 100% load.
I also already pointed out how you can add extra padding (to the padding they already include) by increasing utilizations to 100% and 16 hours per day, and you can add another 100W for good measure, if it will make you feel better - though it is not needed.
Sure you can manually research if so inclined. We used to do it in the shop every time, until I found Outervision's calculator and realized (1) how flexible it is and therefore (2) how accurate it is compared to all other calculators that are much less flexible.
Adding an extra 33% won't hurt anything (except maybe, your budget). But it is still WAY overkill. Even Outervision's conservative calculator ensures an underpowered supply will never be recommended.
Don't forget too that when calculating PSU needs (either manually or with a calculator) you always add up the maximum demands of all the components and use that as your
minimum size. HOWEVER, it extremely unlikely the CPU, GPU, motherboard, each RAM stick, all drives, and all connected USB devices will ever draw maximum power at the exact same point in time. Even during artificial stress tests, which are designed to max out performance and demands, that does not happen. This means, even though the supply is sized to support every device demanding maximum power at the same point of time, the vast majority of the time the demand will be significantly less.
You can follow the link in my sig to see if I might have a little experience in this area.