XFX 850w PSU enough for 970 sli and i7 4790k

It is very much overkill. It's recommended to use a 750W for TWO GTX970s, so if you plan on adding one later in SLI, you can go ahead and get it. Otherwise, 550-650W will suffice.

The less of the 850W you don't use, the less efficient your power supply becomes. The closer to the spec, around the 750-800W range, you will achieve better efficiency.

Otherwise, that's excess heat generated and wasted electricity... if that matters to you.
 
The first paragraph spadam states is correct, the second and third are wrong.

The efficiency difference between 20%-50%-80% usage on any Bronze or above rated PSU is tiny, only if you are using less than about 10% of your PSU's capabilities will the efficiency be noticeably effected.

What he says about heat is even more wrong. On quality PSU's there should be little difference in heat produced between say 650w and 850w, its actually possible for the 850 to run cooler but even if it did it would be insignificant.
 
I do agree with you that it is a, somewhat, miniscule amount, in the end. I do want to clarify that I am NOT meaning that your PSU is trying to produce 850 watts all of the time and if you're only using say 650W then the excess 200W is wasted.

I AM saying that the efficiency curve does go down the less you use, and there is a sweet spot. So yes, when your efficiency goes down it will affect your heat output and possibly your electric bill (but honestly you probably won't notice).

Take this for example: These numbers I have made up but hear me out.

Say your power supply can achieve 80% efficiency at 850W. At the wall, it will pull about 1062W. The difference is 212W, where it is waste and in the form of heat.

So say this same power supply can achieve 70% efficiency at 650W. AT the wall, it will pull about 928W. The difference is 278W, where it is waste and in the form of heat.

If you attain the BEST efficiency at the most optimal range of your power supply, you will cut down on heat AND waste of electricity. Also keep in mind, these are values at FULL LOAD, whereas most home computers are sitting IDLE. Therefore, your efficiency can be cut even lower. So I again recommend, do not buy a power supply that excessively exceeds your power requirements unless you plan later to add components.