(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. GPU starting at almost idle should compensate for the overall power requirement, but still factor it into the rail calculations