The GTX 980 produces 8.98% less wattage of heat than the 290X under load. (Almost all wattage used gets converted to heat)
http://images.anandtech.com/graphs/graph8526/67755.png
Also, on playing the game vs. a standard benchmark:
"Because we measure from the wall, this test means we’re seeing GPU power consumption as well as CPU power consumption, which means high performance cards will drive up the system power consumption numbers merely by giving the CPU more work to do.
For that reason, when looking at recent generation cards implementing GPU Boost 2.0 or PowerTune 3, we prefer to turn to FurMark as it essentially nullifies the power consumption impact of the CPU."