Why can't I just use a 1200W PSU for dual 4090s?

A 1200W PSU is the bare minimum and assumes no transient spikes, system overhead, or future upgrades. GPUs can spike 300–400W above their TDP for 10–100ms during heavy compute ramps. A 1200W supply has no headroom; a 1500W–1600W supply gives you margin for stability. If you plan any future changes, go larger.

Is 15A the hard wall for my AI rig?

Yes, in most US homes. A standard wall circuit is 15A @ 120V = 1800W absolute maximum. Your PSU should not exceed 1500–1600W to stay safely below that ceiling and account for other devices on the circuit. Check your breaker panel or call an electrician if unsure.

Dual PSUs are not necessary for most homelab setups. A single large (1600W+) PSU is cheaper, simpler, and more reliable. Dual PSUs are useful when building a production workstation that cannot tolerate power loss, or when upgrading a system designed for a smaller supply. For local inference and hobby builds, one high-capacity supply is the right choice.

What's the difference between TDP and actual power draw?

TDP (thermal design power) is NVIDIA's maximum expected heat dissipation under sustained load. Real peak power can spike 10–20% higher during transient workloads. For safe PSU sizing, always assume real peak draw exceeds TDP and add headroom (typically 150W–200W above total TDP).

Can I use a gaming PSU for AI workloads?

Yes, as long as it meets the wattage requirement and comes from a reputable manufacturer (Corsair, EVGA, Seasonic, etc.). Gaming PSUs and AI PSUs are built to the same standards. The only difference is the label. Size by the math in this guide, not by the PSU's intended use case.

PSU Sizing for Multi-GPU AI Rigs: The Wattage Math That Keeps Your House Safe

An undersized power supply in a multi-GPU AI rig is not a performance problem — it is a safety problem. When a PSU begins to struggle under sustained load, it does not degrade gracefully. It trips early, halts your training or inference run mid-stream, or in the worst case, fails under thermal stress. At the circuit level, an undersized rig can overload a standard 15A home wall circuit and trip the breaker repeatedly — or worse, overheat the wire in the wall.

This guide gives you the math to size a PSU correctly, once, and never relitigate it. It covers transient-spike headroom (the reason a single RTX 5090 needs far more than 575W), the electrical ceiling that most US builders forget (the 15A circuit limit), and specific wattage budgets for the rigs most people actually build.

Why is headroom not optional?

Here is why PSU sizing for multi-GPU is non-negotiable: GPUs do not draw their TDP steadily. They spike.

A TDP (thermal design power) is NVIDIA’s maximum expected heat dissipation under sustained compute load. Real peak power draw — the momentary spike when all cores ramp and clock simultaneously — can exceed TDP by 10–20%. For the RTX 5090, which carries a 575W TDP, peak transient draw can touch 650–700W. For a dual 4090 setup, where each card has a 450W TDP, simultaneous peak draw is not 900W — it is closer to 1,050–1,100W when both cards spike in phase.

An undersized PSU has no margin for these spikes. When the power draw exceeds the supply’s capacity, the supply’s protection circuit cuts the voltage. This shows up as system instability: brief freezes, kernel panics, or unexpected shutdowns that are maddeningly hard to debug because they happen under real load, not in idle testing.

The fix is simple: add 150–200W of headroom above the sum of your GPUs’ TDP, plus your CPU, storage, and motherboard overhead. This is not paranoia; it is standard practice in data centers and the reason power infrastructure budgets for transient load. For a homelab, it means never building to the edge.

What is the 15A wall-circuit ceiling?

Most US homes run standard 15A circuits (or 20A in newer construction). At 120V, that is a hard ceiling:

15A circuit = 1800W absolute maximum
20A circuit = 2400W absolute maximum

Your PSU’s maximum load should not exceed 1500–1600W on a 15A circuit or 2000–2200W on a 20A circuit. The rest of your headroom protects against wire heating, voltage sag, and other devices on the circuit (refrigerator, office light, etc.).

If you have not checked which circuit your machine is on, check your breaker panel or ask an electrician. If you are uncertain, assume 15A and size conservatively. Many builders ignore this limit and end up chasing breaker trips that have nothing to do with the PSU itself — the fault is the wall circuit.

For the detailed load calculator and to verify your specific build, see the PSU Calculator Tool.

Transient spikes and thermal margin

Here is the second reason headroom matters: efficiency curves. No PSU is 100% efficient. Most are rated 80–92% efficient at 50% load, but efficiency drops at the very high end. A 1600W PSU delivering steady 1550W is running at the edge of its efficiency curve and heating up. The same PSU delivering 1300W sustained has headroom and runs cooler, with lower ripple and better stability under spikes.

For multi-GPU rigs, the rule of thumb from the homelab community (r/LocalLLaMA, r/buildapc) is:

Single GPU: TDP + 300W headroom
Dual GPU: sum of TDP + 300–400W headroom
Triple+ GPU: sum of TDP + 400–500W headroom

This is not a guarantee, and you should always verify with the tool below. But it is a safe starting point that accounts for transient spikes, CPU overhead, and the efficiency drop at max load.

Per-build wattage math

Here are the most common multi-GPU AI rigs and their PSU requirements:

Single RTX 5090

RTX 5090 TDP: 575W
CPU (e.g., Ryzen 7 9700X3D): 120–160W
Motherboard, storage, fans: 50–80W
Total sustained estimate: ~760W
Transient peak estimate: ~900–950W (5090 can spike to 650–700W)
Recommended PSU: 1600W

The RTX 5090 is the first consumer card to truly demand this tier. A 1500W PSU is cutting it close and leaves little margin for future upgrades. Go 1600W if you intend to keep the rig running for 2+ years.

Dual RTX 4090

2× RTX 4090 TDP: 450W × 2 = 900W
CPU (e.g., Ryzen 7 9700X3D): 120–160W
Motherboard, storage, fans: 50–80W
Total sustained estimate: ~1100W
Transient peak estimate: ~1300W (both cards can spike to 500W+ each)
Recommended PSU: 1500W–1600W

A 1200W PSU is the absolute minimum cited in community builds and has no safety margin. If both cards spike simultaneously, you risk instability. The 1500W–1600W range is the practical sweet spot and leaves room for CPU overclocking or future GPU additions.

Dual RTX 3090 (or RTX 3090 + RTX 3090 Ti)

2× RTX 3090 TDP: 350W + 350W = 700W
CPU (e.g., Ryzen 7 5700X): 105–140W
Motherboard, storage, fans: 50–80W
Total sustained estimate: ~870W
Transient peak estimate: ~1050W (3090 can spike to 400W+ each)
Recommended PSU: 1200W–1300W minimum, 1500W ideal

Community guidance (r/LocalLLaMA, r/buildapc) cites 1000W as the bare minimum for this setup. However, 1000W offers zero headroom and is only safe if your electricity is perfectly stable. In reality, most builders use 1200W–1300W; the truly risk-conscious go 1500W and gain headroom for CPU upgrades or concurrent workloads.

Master PSU sizing table

Build	GPUs	Sustained Load	Transient Peak	Minimum PSU	Recommended PSU
Single RTX 5090	1× 5090	~900W	~950W	1200W	1600W
Single RTX 4090	1× 4090	~650W	~750W	1000W	1200W–1500W
Dual RTX 4090	2× 4090	~1100W	~1300W	1200W	1500W–1600W
Dual RTX 3090	2× 3090	~870W	~1050W	1000W	1200W–1500W
Dual RTX 3090 + NVLink	2× 3090 + bridge	~870W	~1050W	1000W	1200W–1500W

All figures are sustained + transient estimates, not independently measured by LocalRig. Actual draw depends on CPU, cooling, memory, and overclocking. Always verify with the PSU Calculator Tool and your specific hardware bill of materials.

Does efficiency rating matter at scale?

For rigs running 24/7 (LLM serving, continuous training), the PSU’s efficiency rating becomes a real cost. A Gold-rated PSU (typically 90%+ efficiency) costs more upfront but saves electricity over years of operation. A Bronze-rated PSU (80–85% efficiency) is cheaper but wastes more power as heat.

For a hobby rig running a few hours a day, efficiency rating is a minor factor. For a production workstation or a multi-GPU inference server, it is worth the $50–$100 premium for Gold or Platinum efficiency. This is not a substitute for sizing headroom — do the wattage math first, then optimize the efficiency rating.

Do I need a UPS?

If your AI rig must not drop mid-inference or training, a UPS (uninterruptible power supply) is not optional. See the best UPS for home AI servers for full guidance.

For multi-GPU rigs, PSU + UPS sizing is compounded. A 1600W PSU drawing 1300W sustained needs a UPS that can deliver at least 1300W for however long you need — typically 5–15 minutes to shut down gracefully or migrate to cloud. This gets expensive fast (a 2000W UPS with 15 minutes runtime is $1500+), so UPS is usually reserved for production or high-stakes workloads, not hobby builds.

The fire-adjacent risk: Per the LocalRig personas doctrine, Persona 4 (researcher/grad student) and Persona 5 (open-source maintainer in a shared space) both run undersized PSUs as a known fire and facility risk. Undersized supplies under multi-GPU load produce heat stress and can trip breakers repeatedly, creating conditions for arc faults and wire overheating in walls. If you are building a rig for a dorm, shared lab, or multi-tenant space, the PSU safety math is non-negotiable, not optional.

Buying a PSU: the checklist

When you have settled on the wattage, here is what to check:

Reputable brand. Corsair, EVGA, Seasonic, be quiet!, and Thermaltake are the safe bets. No-name supplies at half price are not worth the risk.
Modular or semi-modular. Cables you do not need should disconnect. This saves space and cooling airflow, and it matters in tight builds.
80 Plus Gold or better (if budget allows; Bronze is acceptable for hobby rigs).
10-year warranty minimum. A good PSU carries a decade of coverage because it should last that long.
Verify your wall outlet and breaker. If you are unsure whether it is 15A or 20A, check the breaker panel or call an electrician before buying.

For specific product picks at each wattage tier (1200W, 1500W, 1600W), search Amazon and eBay:

Build-planning integration

For a full multi-GPU build checklist — not just the PSU but cooling, mounting, network, and UPS — see the dual RTX 3090 build guide. For the logic of whether a multi-GPU setup is the right choice for your workload (capacity vs. speed), see two RTX 3090s vs. one RTX 4090 and the local-vs-cloud break-even.

To verify your full build’s power budget step by step, use the PSU Calculator Tool, which estimates power draw by component and compares it against your circuit and PSU capacity.

Bottom line

PSU sizing is the one place the homelab community never soft-pedals, and for good reason. An undersized supply can fail thermally, cause instability under load, or worse, overheat the electrical infrastructure in your walls. Add headroom — 150–200W above your GPU TDP sum — and buy from a reputable brand with a long warranty. It is cheap insurance against a $5,000+ rig failing mysteriously or creating a safety hazard.

If you are building a dual-GPU rig, do not settle for a 1200W PSU and hope it works. If you are running a rig on a 15A circuit, respect the 1500–1600W ceiling and verify your breaker does not trip under load. The PSU is not the place to skimp, and the math is not complicated — just unhurried.