The European Union, since 2014, has a mandatory regulation about energy efficiency in power supply units. This is the R. 617/2013, for any power supply sold in the European market. It's an example of rationalization about power consumption, trying to establish limits and at the same time promoting better engineering efforts, quality and responsibility in energy consumption.
The user could be lastly tricked if some manufacturer sells a non efficient power supply. After using specialized equipment, to identify power supplies that at all comply with their specifications is possible. For instance a 500W power supply with 50% efficiency, wastes 250W in heat or worse (radiation). If the user needs 250W, the total power consumption would be 500W. With 125W of needs, 250W and so on. The electricity bill obviously reflects those consumptions, and with long periods of time like 5 years, the difference could be revealing.
The EU regulation serves well to see a bit of reason in the wild. The government must not allow anachronistic engineering specifications where the state of the art already allows efficiencies of more than 90%. If this is possible, then, how some manufacturers are below these limits? Cheap pieces? The circuit design normally is what matters most.
We would like to see more regulations like this, not only for hardware but for software, as it forces the hardware to consume more. Internet browsers, as an example, going for insane amounts of RAM for seeing a bit of text and graphics, shouldn’t be neither allowed. Or smartphone operating systems like Android using blatant techniques to consume memory —also— with caching operations and active processes. These practices are really lamentable in a world where the energy consumption causes environmental toxicity and waste, or resource exhaustion.
Lies in the specifications are another common reality to avoid quickly.
Before buying a power supply always check its efficiency, and while other regulations aren't here, also the brand reputation because perhaps the specifications are very unrealistic.