The system idle results were all much the same, with just 6 watts separating the most power hungry configuration to the most efficient.
Measuring system power consumption reveals that the R7 260X used roughly the same amount of power as the GTX 750 Ti, which is a great result for AMD given that its GPU provided similar performance overall. The R7 260X was faster than the GTX 750 so its 11% increase in consumption can easily be justified.
The R7 265 used 21% more power than the GTX 750 Ti, though it was on average 13% faster so those aren't bad figures for AMD. Meanwhile, the R9 280 and GTX 760 consumed roughly the same amount yet the AMD GPU was faster and the R9 290 pulled 19% more power than the GTX 770, but it was also roughly that much faster.
AMD only really loses out on efficiency here with the R9 290X, which consumed 20% more power than the GTX 970 but was only able to deliver similar performance.