Every device in your PC requires a certain wattage to function. A Hard Drive usually takes 15W of power. A NVIDIA GPU can take ~400W. When buying a PSU, it must be able to support the cumulative wattage of all your devices.
Efficiency Rating
The consumption:return rate of your PSU. From the watts that your PC draws from AC and to how much of it is converted into DC current for your components. 80+: 80% Efficiency 80+ Bronze: 82% Efficiency 80+ Silver: 85% Efficiency 80+ Gold: 87% Efficiency 80+ Platinum: 90% Efficiency 80+ Titanium: 92% Efficiency It is impossible to have 100% Efficiency as energy will always be lost to Thermal Energy It is all about how much money you save in the long run.
Recommended PSU Rating
Always get atleast 550W power supply, and purchase power supplies that have make 50% more power than you need. All power supplies produce less wattage over time, so if you want a long lasting power supply, buy the higher wattage ones.