No PSU's are 100% efficient. A PSU's rated efficiency is based on how efficient the PSU is in transforming the AC power from the wall in to the DC power that your components can use. 80 Plus is an organization that tests and certifies PSU's for manufacturer's and groups them according to their efficiency.
https://youtu.be/P2rjxK9aZ24?t=643Higher rated PSU's consume less power because they use better quality components that are more efficient. e.g. In converting a 500 watt load, a PSU rated at 90% efficiency will consume 555 W from the wall vs a PSU rated at 80% will consume 625 W from the wall for the same 500 W load. The rest is wasted in heat.
How efficient a power supply is also depends on the percentage of the rated load being outputted, with most PSUs running at maximum efficiency around 50% load. Better quality PSU's are able to keep their rated efficiency for a higher percentage of the maximum output. Better quality dual conversion PSU's also convert the entire energy into a 12V single rail and then uses VRM to generate 5V and 3.3V. The cheaper PSU type converts using different taps in the transformer for each voltage and use the 5V line to regulate the voltage, instead of the 12V line like the dual conversion models. In mining, that causes voltage instability from not loading the 5V line.
As far as for mining, I would stick to Gold or higher rated PSU's from EVGA, Superflower, Seasonic or Corsair. Another good less expensive option is using a server power supply, but those tend to be much louder and require using a breakout board.