Step 1: Find the manufacturer-reported wattage for your power supply.
Step 2: Divide your power supply wattage by 60 to obtain the number of 60-watt light bulbs your power supply is equivalent to.
Step 3: An average 60-watt light bulb emits about 800 lumens of light. Multiply the value obtained from step 2 by 800 to obtain the number of lumens your power supply is equivalent to.
Step 4: Realize that, just as with humans, power draw determines how fast and powerful your PC is. So it's not just your power supply that is several thousand lumens bright, but your whole computer!
Step 5: Find the nearest computer geek and explain how powerful your computer is, in lumens, as well as the process you went through to obtain your measure of performance. Try to keep a straight face at their reaction.
Here's the brightness of my PC:
1) My power supply is reported as a 500 watt PS
2) My PS is therefore equivalent to 8.3 lightbulbs
3) That means my computer's brightness is 6640 lumens
4) MY COMPUTOR IZ R FAST
I'm not going to explain how each step contains a hilariously bullshitty figure resulting in the most bullshitty measure of performance ever invented. I'll leave that as an exercise to the reader.
(I learned that from lazy textbook writers. ;))