Is this what the efficiency rating means or does the efficiency rating mean something else?
Here's my understanding of "efficiency". If you look at the spec's for power supplies, you'll find two
efficiency ratings. One for PFC and one that's normally just called "efficiency". IMO, you can't use either to judge how noisy a powersupply will be.
Like others have said in this thread PFC "efficiency" is a measure of how well the AC is transfered from the power company to your PSU. I won't repeat the other stuff that has been said, but I can give you this situation. Imagine your CPU is connected to a UPS
to protect your CPU from surges, blackouts, etc. During a blackout, a computer with Active PFC will drain the battery less quickly than one with Passive PFC. Y? Because Active PFC power supplies are more efficient (usually 90-95%) at handling the power from your UPS. Passive PFC will usually get you about 65%-75%.
The other kind of efficiency measures how well the AC (once at the power supply) is delievered to the electronics. As was already said, the wasted power is converted to heat.