The diagram below will show what motherboard an XBOX 360 has and the PSU ratings.
For the fat consoles, the PSU's are forward compatible. The later consoles require less power, so we can use the older PSUs. The console will only draw the power it needs. If the console requires 203w and we only supply 150w (older console, newer PSU), then this is problematic. The opposite is fine (newer console, older PSU). The cut outs prevent us from using the wrong one.
For the fat consoles, the PSU's are forward compatible. The later consoles require less power, so we can use the older PSUs. The console will only draw the power it needs. If the console requires 203w and we only supply 150w (older console, newer PSU), then this is problematic. The opposite is fine (newer console, older PSU). The cut outs prevent us from using the wrong one.
