Unforunately it's not so straightforward with most PSU
today, in that they spec nonconcurrent, nonsustainable peaks
per rail that aren't additively going to provide the total
wattage the math suggests.
It's been that way for quite a while though, for example the
labeled, combined 3.3V+5V rating of a PSU might be 180W, but
the label might also claim 35A @ 5V and 30A @ 3.3V, which
additively would suggest an unrealistic 274W.
That is because there are parts of the PSU that are shared between
multiple output rails and the shared parts may put a more restrictive
limit on the combined ratings than the individual circuits put on the
separate rails.
Think of it as roads. You have a main road that can carry 10 cars per
minute. This road feeds into 3 side roads than can each carry 5 cars
per minute. Obviously the side roads put a limit on how many cars can
run on each of those roads, but you can't expect to fill all of the
side roads at the same time.
Likewise you can put high loads on any of the separate outputs from a
PSU, but if you try to max out multiple outputs you will bump into a
limit on the combined power. Often there are several combined
limits. I have seen the 3.3 and 5 volt outputs combined with a total
rating and then there is a total limit for the entire supply that is
less than the sum of the combined ratings.
If they built a PSU that could max out all the outputs at the same
time, it would either be much larger and more expensive, or it would
have fairly small individual outputs.
