Is 850W enough for an RTX 4090 + Ryzen 7 9800X3D? (measured, not TDP)

Yes — 850 W handles this combination with room to spare.

noteSustained load: 616 W (Ryzen 7 9800X3D 150 W measured peak + GeForce RTX 4090 391 W measured gaming + 75 W rest of system) = 72% of a 850 W unit.
goodSustained load is 72% of the unit's rating, and even the worst measured spike total (767 W) stays under 850 W.
noteSpike check: 150 W CPU + 542 W GPU spike + 75 W = 767 W momentary worst case — within the unit's rating.

The math, with sources

GPU sustained draw: 391 W — GeForce RTX 4090 measured while gaming (TechPowerUp — GeForce RTX 5050 review, Power Consumption charts (July 2025 test suite)) — its rated board power is 450 W.
GPU transient spikes: 542 W — GeForce RTX 4090 spikes to ~542 W for milliseconds (TechPowerUp — GeForce RTX 5050 review, Power Consumption charts (July 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: 616 W — 150 + 391 + 75 = 616 W of steady load.
With 15% headroom: 708 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: 767 W — CPU peak + GPU spike + rest = 767 W for milliseconds. The recommendation also covers this outright, so even a pre-ATX-3.0 unit is not spike-tripped.
Recommended: 850 W — The smallest standard size covering both 708 W (sustained + headroom) and 767 W (spike total).

AFFILIATE_SLOT: tested 850W ATX 3.x unit picks will be listed here. No links yet — added only once real recommendations exist.