There is a certification for PSU efficiency, basically a PSU has to draw more power than the components need. The higher the efficiency, the better the certification. There are 80 + Bronze, Silver, Gold, and I think Platinum and Titanium or something. The higher on the list, the closer to max theoretical efficiency. There just used to be 80+ where it was 80% but now something like the 80+ Titanium is like 92 or something like that.
Ex: a 50% efficient PSU will draw 600W to power 300W. So your bills are high, stuff's hot, etc. A perfectly efficient one (obiously non-existent) will draw 300W.
https://en.wikipedia.org/wiki/80_Plus
So the bottom line is, you don't have to get a 1200W 80+ Titanium PSU for a normal PC, but just getting any old 500W (or even a good one) may not be ideal for a hardcore gaming machine.