How can i know the watts of my computer's Power Supply Unit (PSU)?

[pranavhait]

on the sticker of the psu written;
+5V = 16A MAX
+5V(SB) = 2A MAX
-12V = 0.5A MAX
+3.3V = 20A MAX
+12V = 15A AMX
I going to buy a graphic card which require 300 watts of power supply(like amd/ati 5450 or nvidia geforce 210 or some budget gpu), will it work

 

[Unolocogringo]

5x16=80 watt
3.3x20=66watts
12x15=180watts.
80+66+180=326watts max
Most power supplies can not deliver the max amount on all voltage rails at the same time.
So yours is probably an older 300 watt power supply.
The 12v rail is the most important one in newer computers.
The cards you listed will work on your power supply.