According to the official specs, that prebuilt PC can come with:"up to 450W" PSU,
link:
https://www.lenovo.com/gb/en/desktops-and-all-in-ones/legion-desktops/legion-y-series-desktops/Legion-Y520-Desktop/p/99LE9Y50275
In other words, if the specs would be Core i3/i5 with GTX 1050 Ti, you'd see 280W PSU in it since GTX 1050 Ti is 75W GPU and rest of the system would be at 150W or so, making the max power draw about 225W or so, where 280W PSU would be enough.
But if the specs would be Core i5/i7 with GTX 1060 3GB, you'd see 450W PSU in it. GTX 1060 3GB is 120W GPU and rest of the system would be at 200W or so, making the max power draw about 320W or so, where 450W PSU would be enough.
As far as prebuilt PCs in general go, they all are overpriced and in most of times, you can build better performing PC with the same amount of money. Also, PSU build quality in the prebuilt PCs is questionable and if i'd had prebuilt PC, 1st thing i'd do would be replacing PSU for a known good quality unit.
Though, buying all the components by yourself means that you need to assemble your PC by your own, including OS install (or pay someone to assemble it for you). PC assemble may be viewed as downside, especially if you don't know how to put it all together. But PC enthusiasts (myself included) love to assemble their own PCs.