In an effort to help new system builders decide on components for their build, ASUS has created a Power Supply calculator – put in your hardware and out pops a number of power supply watts that should be used for the system.
For example, we put in an Intel i7-4770 system with two NVIDIA GeForce GTX 780s, and the system config came out to 1050 W, which offers a good headroom for such a configuration. In reality, a 750W would be ok for this setup, but 1050W would be plenty.
I always liken a power supply to a car – if you have a car that does 80mph max, and you always drive around at 70mph, it will cough and splutter and fail sooner. If you get a car that does 150mph max and drive around at 70mph, then all will be fine for longer.
Have a go, see if you have the correct power supply for your build!