Worked example: dual RTX 4090 AI workstation
The default build uses 2 x RTX 4090 at 450 W each, so GPU TGP is 900 W. At 100% AI load, sustained GPU planning draw stays 900 W.
The rest of the system is 170 W CPU + 60 W motherboard + 2 x 8 W drives + 6 x 3 W fans + 10 W pump + 25 W USB and PCIe allowance = 299 W.
Sustained system draw is 900 W + 299 W = 1,199 W. With 5% aging reserve and a target of keeping sustained load at 80% of PSU capacity, target capacity is 1,199 x 1.05 / 0.80 = 1,573.7 W.
The transient check is 900 W x 1.20 + 299 W = 1,379 W. The larger planning number is 1,573.7 W, so the calculator rounds up to a 1,600 W PSU. Sustained load on that PSU is about 1,199 / 1,600 = 74.9%.
How the AI PSU wattage estimate works
The calculator starts with GPU total graphics power multiplied by GPU count and sustained load percentage. It then adds CPU package watts, motherboard allowance, storage, fans, pump, USB, and PCIe accessory load.
The recommendation is the larger of two checks: sustained watts with aging reserve divided by the target PSU load, or GPU TGP with transient reserve plus the rest of the system. The result is rounded up to a common PSU size.
Why multi-GPU AI builds need more margin
AI workloads can run sustained GPU load longer than gaming bursts, and multiple cards concentrate heat and cable current. A build that technically boots on a smaller PSU may still be noisy, unstable, or uncomfortable for long unattended jobs.
What PSU size for two RTX 4090 GPUs?
With the default 170 W CPU and accessory assumptions, two RTX 4090 cards produce a 1,199 W sustained estimate and a 1,600 W rounded recommendation. A lower-power CPU, power-limited GPUs, or measured draw can reduce the number, but the cabling still needs careful verification.
How to use measured wall power
If you already own the system, measured wall power is useful but includes PSU losses. For planning, use component DC draw when possible, then compare wall measurements under the exact training, inference, rendering, or stress workload you care about.
What to do next
After the wattage screen, verify case clearance, airflow, individual cable ratings, connector type, branch circuit capacity, UPS size, and the exact add-in-card vendor specs before buying a PSU.
