Enter the power of your CPU, graphics card and other parts to estimate the total load and the power supply size to buy.
Note: use the graphics card's total board power (TBP) rather than chip TDP, and check the card maker's recommended PSU. High-end GPUs can draw short power spikes well above their rating, which is why 20–30% headroom is common.
Add 20–30% above the estimated load. It covers short power spikes from modern graphics cards, keeps the PSU in its efficient range (usually around 50% load) and leaves room for upgrades.
For the graphics card use the total board power (TBP) from the manufacturer, not the chip's TDP. Many CPUs can exceed their TDP under boost, so check the maximum package power for high-end models.
With a 125 W CPU and 75 W for other parts the load is about 485 W; with 30% headroom a 650 W PSU is a good fit.
Not much. A good PSU is efficient across a wide range, but buying far more than you need mainly costs more money.
It is an efficiency rating. Higher levels (Gold, Platinum) waste less power as heat but do not change how many watts the PSU can deliver.