at the time the P3s were released, the PSU at that time were only 300-350Watts so that is the common that you will see. BUT, there is no problem even if you use a PSU of more than 350W, you can use 1000Watts if you want, it wont be a problem
yung amin dito na P3 800, nasunog ang PSU before kasi matagal na,pinalitan ko ng 500Watts