Power supplies smooth, filter, and rectify electricity that comes from wall outlets into a form that PCs can use. You can read more about that here. In doing so some of that electricity is lost in the form of heat. Older power supplies used to do so at much poorer rates like 60%-70%. That means out of the 100 watts your putting in 40%-30% was being wasted. This is an inescapable effect of physics for now. But some of that waste can be mitigated by using higher quality components more conductive paths for the electricity to flow and overall design changes. That why the 80 Plus certification was developed.
The 80 Plus Certification was developed to promote more efficient power supplies. I’ve included a chart above that gives you the efficiency at certain load points. For example the 80 Plus Bronze level at 20% load 82% of the electricity coming from you wall should be used to power the components of your PC and only 18% is lost. Load essentially mean if your power supply is rated at 100 watts your computer is demanding 20 Watts it is at 20% load. But as you can see in the chart there is no efficiency rating for less than 20% which mean at lower loads you power supply may be less efficient that is why it is important to buy a power supply with proper wattage. You can read more about that here. The price for 80 Plus rating don’t necessary follow the rating system for example you may find a 600 watt 80 Plus Gold for less then a 600 watt 80 Plus bronze. Many factor go into the pricing of a PSU like branding, components, accessories, modularity, and cosmetics. The important thing is to verify the PSU certification you can check a PSU 80 Plus rating here.
How important is efficiency? That is a tricky question for most consumers who may only use their PCs 3 hours or less a day it probably won’t make much difference on their power bill. But business and servers who run computer for 8-10 hours a day or even 24 hours would definitely benefit from more efficiency. The difference is significant from 80% efficiency to 90% means 10% less draw from the wall. If you are running a 500 watt server that mean you would draw 50 watts less on average. That would equate to nearly 438 kWh or Kilowatt hours a year at an average electicical rate of .12 per kWh that is about 52.56 dollars a year. That would make a significant difference if you had multiple machines. Also that would mean you wouldn’t have to pull that 50 watts out of the air when cooling that room. Efficiency is a tricky thing but if you have hundreds of users and dozens of servers it is an important consideration. And like i said a platinum power supply may not be much more expensive than a bronze so shop around and buy the right power supply.