PSU calculators work by taking a list of your components (CPU, GPU, drives, fans, cooling loops) and applying a formula to estimate the total power draw. Most quality calculators then add a "safety buffer" (usually 20-30%) to recommend a wattage rating that ensures your PSU isn't running at 100% load.
[ P = V \times I ]
Modern high-wattage PSUs are “single +12V rail” designs, simplifying load balancing.
If planning 5+ years of use, add 20% to compensate for capacitor aging: