Is 850W enough for an RTX 5090 + Ryzen 7 9800X3D? (measured, not TDP)

No — 850 W is not enough for this combination.

noteSustained load: 812 W (Ryzen 7 9800X3D 150 W measured peak + GeForce RTX 5090 587 W measured gaming + 75 W rest of system) = 96% of a 850 W unit.
problemSteady load alone uses 96% of the unit's rating — no margin for heat, aging, or anything extra. Recommended size: 1000 W.

The math, with sources

GPU sustained draw: 587 W — GeForce RTX 5090 measured while gaming (TechPowerUp — RTX 5090 Founders Edition review, Power Consumption charts (Jan 2025 test suite)) — its rated board power is 575 W.
GPU transient spikes: 644 W — GeForce RTX 5090 spikes to ~644 W for milliseconds (TechPowerUp — RTX 5090 Founders Edition review, Power Consumption charts (Jan 2025 test suite)). Spikes trip older PSUs that averages never show.
CPU peak draw: 150 W — Ryzen 7 9800X3D measured under heavy all-core load (TechPowerUp — Ryzen 9 9950X3D review, multi-threaded power chart (Blender, CPU-only)). Games draw less, but renders, encodes and shader compiles hit this.
Rest of the system: 75 W — Motherboard, RAM, drives, fans and USB devices — a fixed allowance.
Sustained total: 812 W — 150 + 587 + 75 = 812 W of steady load.
With 15% headroom: 934 W — Sustained x 1.15 — margin for capacitor aging, heat, and staying near the efficiency sweet spot. (The sustained figure is already conservative: CPU all-core peak and GPU gaming peak rarely coincide.)
Spike check: 869 W — CPU peak + GPU spike + rest = 869 W for milliseconds. The recommendation also covers this outright, so even a pre-ATX-3.0 unit is not spike-tripped.
Recommended: 1000 W — The smallest standard size covering both 934 W (sustained + headroom) and 869 W (spike total).

AFFILIATE_SLOT: tested 1000W ATX 3.x unit picks will be listed here. No links yet — added only once real recommendations exist.