How many watts do I actually need for an RTX 4080 or 4090 build?

Across the reviewers and build communities we read, an 850W unit comfortably covers most RTX 4080-class single-GPU builds, while 1000W is the common recommendation for RTX 4090/5090 systems or heavy overclocking with headroom for transient spikes. ATX 3.1 / PCIe 5.1 units are designed to absorb power excursions well above their rating, which reduces the need to massively oversize.