How Long Should a PSU Last?
Power Usage The first thing you need to be aware of is the maximum power your PC will require. If you’re going to game with NVidia’s new GPUs here’s a brief overview of their TDPs and their recommended PSU wattage. As a rule, you should aim for a PSU which is rated for at least 20% more than your maximum power draw. This ensures that the PSU is not strained even when you’re running your PC at maximum capacity. Future Upgrades should also be kept in mind, if you have plans for them, as you’d not want to buy a separate PSU for them.
Source: xbitlabs.com