Quick answer: 1200W for dual RTX 3090s, 1500W for dual 4090s, 1600W for dual 5090s, 750W for dual 3060s. Those are the recommended sizes; the bare minimums are 1000W, 1200W, 1500W and 650W respectively, and the connectors matter as much as the wattage — a supply without enough native PCIe or 12VHPWR cables will not boot the machine at all.
Running two GPUs for LLM inference doubles your VRAM — and nearly doubles your power draw. An undersized PSU will crash under load, and a PSU without the right connectors won’t even boot. I’ve built dual 3090 and dual 4090 rigs for local LLM work, and the power supply is the component most people get wrong.
Here are the exact numbers.
NVIDIA GeForce RTX 3090
24GB GDDR6XTwo used RTX 3090s (~$1,640 total) give you 48GB combined VRAM for 70B model inference. Budget 1000W+ for the PSU.
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Wattage requirements by GPU pair
| GPU Pair | GPU Power Draw | CPU + System | Total Draw | Minimum PSU | Recommended PSU |
|---|---|---|---|---|---|
| 2x RTX 3060 12GB | 340W (170W each) | 125-150W | ~490W | 650W | 750W |
| 2x RTX 3090 | 700W (350W each) | 125-150W | ~850W | 1000W | 1200W |
| 2x RTX 4090 | 900W (450W each) | 125-150W | ~1050W | 1200W | 1500W |
| 2x RTX 5090 | 1100W (550W each) | 125-150W | ~1250W | 1500W | 1600W |
These numbers assume a mid-range CPU (Ryzen 7 / Core i7), standard RAM, NVMe storage, and no other major power consumers. If you’re running a Threadripper or HEDT platform, add another 50-100W.
The rule: your PSU should deliver at least 20% more wattage than your total system draw. PSUs degrade over time, and running at 90%+ capacity continuously shortens their lifespan and increases ripple.
Power connector requirements
This is where builds fail. Each GPU needs its own dedicated power cables — never daisy-chain a single cable to power two GPUs.
| GPU | Connectors per card | For two cards |
|---|---|---|
| RTX 3060 12GB | 1x 8-pin | 2x 8-pin total |
| RTX 3090 | 2x 8-pin PCIe | 4x 8-pin total |
| RTX 4090 | 1x 12VHPWR (16-pin) | 2x 12VHPWR total |
| RTX 5090 | 1x 12V-2x6 (16-pin) | 2x 12V-2x6 total |
For dual RTX 3090s: your PSU needs at least 4 separate 8-pin PCIe power cables. Most 1000W+ PSUs include 4-6 cables, but verify before buying. Some budget 1000W units only ship with 3 cables.
For dual RTX 4090s: you need 2 separate 12VHPWR cables. ATX 3.0 or ATX 3.1 PSUs include these natively. Older PSUs require adapters — I strongly recommend against adapters for dual-GPU builds. The fire risk with cheap 12VHPWR adapters is real.
Check NVIDIA GeForce RTX 4090 on Amazon→Buy on Shopee SG→Single rail vs multi-rail
Single 12V rail PSUs are strongly preferred for dual-GPU setups. Multi-rail PSUs split their 12V output into separate rails with individual overcurrent protection (OCP). If one rail can’t deliver enough current for a GPU’s transient spike, the PSU shuts down — even though total wattage is available.
Most modern 1000W+ PSUs are single-rail. Check the spec sheet: look for “Single +12V rail” or a single 12V amperage rating. If you see “12V1” and “12V2” listed separately, it’s multi-rail.
80+ efficiency ratings matter more for dual GPU
With dual 3090s pulling 850W from the wall, PSU efficiency directly affects your electricity bill and heat output:
| Rating | Efficiency at 50% load | Waste heat at 850W draw |
|---|---|---|
| 80+ Bronze | 85% | ~150W |
| 80+ Gold | 90% | ~94W |
| 80+ Platinum | 92% | ~74W |
| 80+ Titanium | 94% | ~55W |
80+ Gold is the minimum I’d recommend for dual GPU. The electricity savings over Bronze pay for the price difference within a year at typical US rates. For a dual RTX 3090 build, a Gold-rated 1200W PSU is the sweet spot.
Check NVIDIA GeForce RTX 3090 on Amazon→Buy on Shopee SG→ATX 3.0 vs ATX 2.x — which do you need?
If you’re running any RTX 40 or 50 series card, get an ATX 3.0 (or 3.1) PSU. These include native 12VHPWR/12V-2x6 connectors and are designed to handle the transient power spikes that newer GPUs produce — spikes that can trip OCP on older units.
For dual RTX 3090 builds, ATX 2.x PSUs work fine. The 3090 uses standard 8-pin PCIe connectors and its transient behavior is well-understood by older PSU designs.
My top PSU recommendations by build
Dual RTX 3090 (LLM inference):
- Budget: Any reputable 1000W 80+ Gold, single-rail — EVGA SuperNOVA 1000 G7, Corsair RM1000x
- Recommended: 1200W 80+ Gold — Corsair HX1200, Seasonic PRIME GX-1200
Dual RTX 4090 (LLM inference):
- Required: 1500W ATX 3.0 with 2x native 12VHPWR — Corsair HX1500i, be quiet! Dark Power 13 1500W
- Avoid: Any PSU requiring 12VHPWR adapters for this build
For guidance on the best motherboard for dual GPU LLM setups and the full multi-GPU build guide, those articles cover the rest of the component stack.
Common mistakes
- Daisy-chaining power cables. One cable with two 8-pin heads was designed for a single GPU. Never use one cable to power two cards.
- Ignoring transient spikes. GPUs briefly draw 150-200% of their TDP rating. A PSU rated exactly at your total draw will trip its protection during these spikes.
- Using adapters for 12VHPWR. Fine for a single card in a pinch. Unacceptable for dual-GPU sustained loads.
- Forgetting the CPU. A Threadripper 7000 can pull 350W alone. Factor your actual CPU into the calculation.
If you’re building specifically for 70B model inference, dual 3090s at 1200W is the proven, cost-effective setup.
NVIDIA GeForce RTX 4090
24GB GDDR6XTwo RTX 4090s need a 1500W ATX 3.0 PSU with native 12VHPWR cables. No adapters.
Affiliate links — we may earn a commission at no extra cost to you. Amazon ships globally; Shopee SG covers Singapore & ASEAN.
Frequently asked questions
Can I run dual RTX 3090s on an 850W PSU?
No. Dual 3090s draw ~850W total system power, and transient spikes can exceed 1000W. You need at least a 1000W PSU, with 1200W recommended.
Do I need an ATX 3.0 PSU for dual RTX 3090s?
No. The RTX 3090 uses standard 8-pin PCIe connectors. ATX 2.x PSUs work fine. ATX 3.0 is only needed for RTX 40/50 series cards with 12VHPWR connectors.
Can I use one power cable with two 8-pin connectors for two GPUs?
Never. Each GPU should have its own dedicated power cable from the PSU. Daisy-chaining can cause voltage drops, instability, and in worst cases cable damage.