You can run some rough numbers using
http://www.tomshardware.com/reviews/radeon-hd-7870-review-benchmark,3148-19.html
with the assumption that the gtx480 and 580 as having equal load consumption and the 480 likely having worse idle consumption. A 480 vs 580 review should confirm the first premise not sure about the second.
These is a 30W difference in idle consumption and a 40W diff in display off idle. Ill use a conservative 30 watts for the idle calc.
Ill use the 7870 load figures (as has been said, if you get a 7850 you may as well oc it) which gives a (conservative) 100w diff.
Assuming a 24/7 on PC, 12 hours gameplay per week and a 0.15p per kwh, it comes out to a conservative estimate of £46 per year.
Thats not factoring in the 10 watt higher 'display off' power of the 580 over the 7850 from the data, or the fact that the 480 was a smidge more power hungry and probably had worse idle power gating than the refined 580, or that the 7850 in the above review had a little more vram to power. If you run dual monitors you also have to consider the raised clocks or if you fancy some altruistic gpgpu action when not gaming, extended time at load.
I will agree that power consumption isnt always an important consideration but depending on usage scenario's it can be and I think definitley is when looking at second hand older highend gpu's.
The kettle argument doesn't entirely hold water either. People like their hot drinks, not using power to heat the water means going without. The same for the other appliances. In the case of the gpu's you are not losing out and are getting equivalent usage (and performance) at lower electricity cost.