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It's not quite that simple because the power supply is designed to adjust its output to keep voltages constant under different current draws, but yes, if it can't keep up with demand then voltages will drop, and that's not ideal long term.
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It's a fairly high quality PSU and would likely cut power long before anything unsafe. 650W is definitely enough. They're covering themselves in case someone uses it with a 16+ core overclocked CPU and complains that it blew out their under-capacity sketchy PSU which this isn't.
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Unless there are 20x HDDs or something crazy like that it's definitely enough. You'd probably want to bump up to 850W if you had something like a Ryzen 9 5950X (16 core) that you were going to overclock. Even with that CPU, it would be enough if you weren't overclocking it.
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Her card draws 310W, but I'm pretty sure from talking to her that the rest of her system never draws more than maybe 200W under severe load, so yeah. I would update the PSU if it were me, because it's a nice GPU, but then I run a 1200W PSU because I overbuild everything. :)
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