1.Buy high-efficient power supplies that are really outputting the amperage it says it does.
These manufacturers include EVGA Enermax,Thermaltake,Superflower,Corsair and avoid the non-famous ones.
2.In case you buy a power supply from unfamous and not that popular manufacturer just make sure you add extra 50-100Watts more than you need just in case the effiency is not good enough for modern gaming machines.
For example if your GPU requires 500Watts add extra 50-60 and buy 600 Watts.
3.Do not trust what you see on the label but look at reviews of other users and actual peromance.For high-end machine its important that you have high-end power supply.The risks of buying not good and powerful enough power supply is to burn the others parts if its protection fails or in the best case scenario to shut down the system.
4.Look at the overall quality of product(perfomance,capacitors,etc) Thanks for the idea/point to @tzzsmk.
You might already know that but just in case as a reminder ;].I wanted to share my non-expert opinion for power supplies and raise the problem of how important one power supply is in a modern high-end gaming machine.Don't underestimate that otherwise you will end up buying a new cpu,gpu,motherboard and whole computer.Also overclocking really shortens your power supply and cpu,gpu lifespan.
Hope you find this useful.Thanks for reading!
Table of nominal and real output of power supplies in Ampers:
200 Watts – 17 Ampers @ 12V
300 Watts – 25 Ampers @ 12V
400 Watts – 33 Ampers @ 12V
500 Watts - 42 Ampers @12V
600 Watts – 50 Ampers @ 12V
700 Watts – 58 Ampers @ 12V
800 Watts – 67 Ampers @ 12V
900 Watts – 75 Ampers @ 12V
1000 Watts – 83 Ampers @ 12V
1100 Watts – 92Ampers @ 12 V
1200 Watts – 100 Ampers @ 12V
1300 Watts – 108 Ampers @ 12V
1400 Watts – 117 Ampers @ 12V
1500 Watts – 125 Ampers @ 12V
1600 Watts – 133 Ampers @ 12V
1700 Watts – 142 Ampers @ 12V
2000 Watts – 167 Ampers @ 12V