Here is Guru3D's power supply recommendation:
GeForce GTX 980 - On your average system the card requires you to have a 500 Watt power supply unit.
GeForce GTX 980 in 2-way SLI - On your average system the cards require you to have an 800 Watt power supply unit as minimum.
If you are going to overclock your GPU or processor, then we do recommend you purchase something with some more stamina. There are many good PSUs out there, please do have a look at our many PSU reviews as we have loads of recommended PSUs for you to check out in there. What could happen if your PSU can't cope with the load is:
Bad 3D performance
Crashing games
Spontaneous reset or imminent shutdown of the PC
Freezing during gameplay
PSU...