In this case, the specification limits the additional power supply to 450 watts. The video card works without any problems, but its performance drops by about 5%. If the power transfer is limited to 400 watts, then the RTX 5090 will lose additional power and provide up to 90% of the performance of the original level.
During the experiments, we also found out that when using only two of the four additional 8-pin connectors (+300 W), the video cards no longer work - the image is not displayed on the screen, and there is no separate message about the lack of power. This is true for both GeForce RTX 5090 and GeForce RTX 5080. At least when it comes to the Founders Edition modifications and the adapter from Zotac that we reviewed.
In general, the ability to fully utilize a top-of-the-line graphics card with certain power consumption restrictions is more of a plus, even despite some performance losses. Although they are more noticeable than, for example, in the case of the RTX 4090, which lost only ~2% when limited from 450W to 350W. However, those who want to get a GeForce RTX 5090 are probably more concerned about the availability of such graphics cards on sale. They are not yet available in Ukraine, but this is not a local logistics issue. Due to the shortage of suitable GPUs, manufacturers are delaying the delivery of video cards, which leads to a shortage.