(Σ (max TDP of all system components) ) + 20%

Rationale:

• PSU works best at 20–80% load
• Component TDP are an average; actual power usage can go way down, but also spike above spec for short bursts
• Brownout is one of the messiest issues to troubleshoot

Additional considerations:

• Check the PSU rail distribution
• If separate rails, make sure the GPU rail can meet GPU's TDP+20%
• If multiple rails, each should meet whatever is connected to it +20%
• If using HDDs or other start-heavy components, factor in the initial power spike (staggered start is also an option). GPU starting at almost idle should compensate for the overall power requirement, but still factor it into the rail calculations