Originally Posted by djriful
PSU doesn't run at 100% efficiency otherwise that's mean exactly 320Watts of yours. This is why, the rating is there for.
Semi-correct conclusion, completely wrong reasoning. A PSU is rated at its OUTPUT power, not its input. That means the PSU can output 320W continuously. However due to power wasted during the conversion from 120VAC to the DC output, it will consume more power from the wall. So at 320W output, it will be using 320/.8 = 400W input from the wall.
The reason I call it a 250W is because it is built to an older PSU electrical standard, ATX 1.3, that puts more power on the +5V and +3.3V rails, which is useless to a modern machine that gets 80% of its power from the +12V rail.