The "80 Plus" certification means that with a load of between 20% and 100%, the PSU must be at least 80% efficient in the conversion process.
For example, let's say all the PC components consume a total of 100 watts at a given moment. Assuming the PSU is 80% efficient when loaded at 100 watts, the PSU itself would consume 25 watts to make up the other 20%, resulting in 125 watts being drawn from the plug. I.e. 80% of 125 watts = 100 watts.
The 80 Plus comes in various levels also - Bronze, Silver and Gold. You'll see a few PSUs with these certifications.
The minimum efficiency requirements for the certifications are as follows, going by the 80plus website:
- 80 Plus - 80% efficient at 20% load, 80% efficient at 50% load and 80% efficient at 100% load
- 80 Plus Bronze - 82% efficient at 20% load, 85% efficient at 50% load and 82% efficient at 100% load
- 80 Plus Silver - 85% efficient at 20% load, 88% efficient at 50% load and 85% efficient at 100% load
- 80 Plus Gold - 87% efficient at 20% load, 90% efficient at 50% load and 87% efficient at 100% load
If you have a few PSU brands and models handy that you're not sure how efficient they are, you can look them up on the 80plus website list.
Unfortunately, as with other products, a more efficient PSU tends to be a lot more expensive than a less efficient model, so it's worth taking into account how often the PC will run, especially continuously around the clock. Also note that a more efficient PSU does not necessarily mean it is more robust or reliable than a less efficient PSU.
Finally, when building a PC for energy efficiency, in my opinion, the parts that make up the PC (motherboard, CPU, HDD, etc.) play a more major part than the PSU efficiency. For example, there is no point in installing a high end graphics card if the person does not play games or use any CAD software, as a high end graphics card can easily consume over 50 watts alone idle.