AIBs have recommended 1000W and 1200W power supplies for their variants of the GeForce RTX 4090. Powered by (up to) 24+4 phases, these GPUs will likely draw over 500W of power under full load, stressing the more robust heatsink designs. It’s unclear whether you’ll lose any performance if you go ahead with a sub-1000W power supply, but you’ll almost certainly run out of overclocking headroom.
I run into system crashes on my GeForce RTX 3080 Ti FE rig when the power slider is pushed past 110%. These issues are more common on the RTX 3090 and the 3090 Ti because of the higher power ceiling. I wouldn’t recommend buying anything less than a 1000W SMPS without even testing the card unless you already have one.
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