Real world? If you run a normal overclocked quad core with a 980Ti you will bearly suck 400W from the wall. If you don't plan to go SLI/CF anything over a 550W psu is just a waste of money and electricity. And please don't tell me, you might want to go SLI in the future. Noone thinking that ever did it.
Here in Germany you have to pay nearly 0,3€/KWh, playing 2h every day with a normal PC, using maybe 300W, you will save 5-7€ a year. The difference between a good bronze and a good gold psu is around 30€. In your use case of 8 years you could have saved 25€ already...assuming you PC only runs 2 hours a day, but it could be much more if you use it more.