Is 1200W enough for an RTX 5090 + Ryzen 9 9950X3D?

Yes — 1200 W handles this combination with room to spare.

noteSustained load: 865 W (Ryzen 9 9950X3D 203 W measured peak + GeForce RTX 5090 587 W measured gaming + 75 W rest of system) = 72% of a 1200 W unit.
goodSustained load is 72% of the unit's rating, and even the worst measured spike total (922 W) stays under 1200 W.
noteSpike check: 203 W CPU + 644 W GPU spike + 75 W = 922 W momentary worst case — within the unit's rating.

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: 203 W — Ryzen 9 9950X3D 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: 865 W — 203 + 587 + 75 = 865 W of steady load.
With 15% headroom: 995 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: 922 W — CPU peak + GPU spike + rest = 922 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 995 W (sustained + headroom) and 922 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.