A PSU’s labeled wattage (e.g., 850W) is the total combined output across all rails, but a poor design might have a weak +12V rail. Always check the +12V amperage. A quality 850W PSU will supply 70A (~840W) on +12V alone.
If buying a GPU with 12VHPWR, get an ATX 3.1 PSU with native 12V-2x6. Use only the cable that came with the PSU (pinouts are not standardized). Inspect the connector monthly for discoloration. computer power requirements