The first thing you need to know about the SMPS Watt rating is that do not get fooled by the Watt rating.
This is a very common mistake.
A good 300 Watt SMPS can easily beat the cheap local branded 500 / 600 Watt SMPS.
When selecting a SMPS, the first thing you should consider is how many amps of power it needs to operate properly. You’ll need at least a 1 amp (12 volt) output to start.
Every single-phase supply contains a voltage/current chart that shows the current output in amperes for each phase. Look for the amps below 12V and make sure they are at least 30 amps for a dual-rail smp.
Both of those amps should have at least a 16A and 18A current.
A good smps can have an output up to 30 amps, which is fine for most systems that do not include many fancy features.
A Current Sense on a 12V Rail will usually give over 35Amps at 12V rail.