If you are, like me, completely hopeless at choosing computer parts, here is a simple test for your choice of power supply. Read the power output table of your new power supply. Here is one from this power supply unit.

I have highlighted some cells to make it easier to read.

Power Output
AC Input Voltage: 100 V - 240 V
Current: 9.5－4.5 A
Frequency: 50 Hz - 60 Hz
DC Output Rail +3.3 V +5 V +12 V -12 V +5 VSB
Maximum Power 20 A 20 A 58 A 0.3 A 3 A
100 W 696 W 3.6 W 15 W
Total continuous power 700 W

This power supply can output 696 Watts of power out of 700 Watts available on one rail alone, the +12 Volts one. This is one of the metrics of a good power supply. Let’s compare it to my current one (no name brand).

Power Output
AC Input Voltage: 100 V - 240 V
Current: 5 A
Frequency: 50 Hz - 60 Hz
DC Output Rail +3.3 V +5 V +12 V (1) +12 V (2) -12 V +5 VSB
Maximum Power 30 A 36 A 19 A 19 A 0.8 A 2.5 A
280 W 450 W 9.6 W 12.5 W
Total continuous power 750 W

This table tells us that the two 12 Volt rails combined can put out 450 Watts of power out of the 750 Watts available. The ratio here is 60% which is quite bad, especially compared to the 99% of the other table.

You power supply should be very good at feeding 12 Volts because the CPU, GPU and other PCI-E will draw from 12 Volt rails throught the motherboard. If your GPU needs a dedicated rail on the power supply then remember to get one with two 12 Volt rails. The total design power (TDP) of your computer should be about half of your power supply rating as there may be spikes of power consumption up to twice the TDP.