I asked a question before, which was the point of the whole OP's
problem, I believe. How would you use a multimeter to determine how many
watts the PS could put out before the system wouldn't start reliably.
Instead of answering the question, ...
Nobody knows nor cares. First wattage for each voltage is
important; not total wattage. Second, label on a power supply that
says 300 watts could also mean the supply is 210 watts or could mean
it is 425 watts. You don't care how many watts because that says
nothing useful anyway. Third, that web site wattage number is how the
naive answer the naive; really provides nothing useful.
A solution means maximizing a load; then measuring four voltages.
Those measurements report what is important for wattage on each
voltage, excessive ripple voltage, and other potential problems - all
in two minutes.
Nobody here is spending $thousands for dynamic loading test
equipment that is required for your question. OP has a system whose
power requirements are mostly unknown. A total wattage number says
little about each voltage AND that does not say where / how wattage
was measured.
The multimeter does what the OP needs - answers his question
definitively. Notice the last word in that sentence. Shotgunning does
not provide the word 'definitive'. Meter can also do what you ask.
But buy or build a small circuit. It is simple with first year
technician knowledge routinely found among Radio Shack customers (with
some wires, connectors, and a few parts), but completely irrelevant to
the OP's problem.
To obtain total wattage consumption, then the meter and a
multiplication can also provide that number. Again, this requires
simple electrical knowledge. Or spend another $20 for a Kil-a-Watt.
But that wattage number still provides zero useful numbers for the
OP's problem - still results in no useful replies here.
Notice the point so little understood by many A+ Certified computer
techs. The relevant wattage is for each voltage. Since one need not
know how electricity works to be A+ Certified, then those techs only
look at two numbers for a supply - dollars and watts. But a power
supply is chock full of essential functions - at least one full page
of numerical specs apply. IOW if you are asking about selecting a
supply, well, long before that, ask for many other important numbers
that manufacturers of inferior supplies often 'forget' to provide -
for good reason. Many supplies sold to naive certified techs are
missing required functions because they don't even know what those
functions are.
But if asking about this supply in this computer - two minutes with
a 3.5 digit multimeter is necessary. This supply in this computer is
the only relevant question to an OP. Total wattage consumption is
irrelevant and mostly useless information, but is too often hyped by
those who don't even know what a power supply must accomplish.
Notice how much knowledge can assist the OP if he only spends two
minutes with a multimeter. Nothing useful provided again because
numbers from the meter are not provided. His replies will only be as
useful as the information he provides. That means two minutes with a
meter.