just to help you understand a little more let me break it down for you.
My system as an example:
I have a ASUS Strix 1070 overclocked as far as it'll go that also has RGB on it so it uses up some good watts
an i7-6700k overclocked to 4.7Ghz at 1.4v (which is quite the power hog mind you)
a Kraken X62 AiO cooler which has a pump that uses watts and 2x140mm fans that also use watts and RGB lights that also use watts
a 140mm case fan with LED lighting that uses watts too
a 1.5tb HDD and a 250GB SSD which are also using watts
an ASUS Gaming Pro Aura motherboard that has RGB lighting around it's edges that also use watts and of course RAM which uses watts as well.
With this whole setup I am using less than 600w. Not sure the exact amount but I'm pretty I would say I'm at most using a total of around maybe 500w max for everything.
But for this example let's say that I threw in some more case fans and some extra LED or RGB LED lighting for the case and I'm using 600w even.
With the addition of another 1070 I would only be adding up to at least 180w with GPU boost doing it's thing. But let's say I manually overclock and push it a bit further. I would be using about 200 -210 watts most likely on this card and an 850w psu would leave me with 30w to spare. but I'm not actually using 600w on everything else and am about 500w at most and more likely I believe I am using around 450w of power out of my 600w PSU. with an 850w PSU after adding another 200w to my initial system for the second GPU I would be essentially using up about 650 -700w out of that 850w leaving me about 150 - 200w of overhead which is perfect and safe for the system. If you are someone who really likes to nickle and dime their electricity consumption in their home and absolutely requires being able to maximize the power efficiency to minimize their power consumption on their power bill to the penny then go with the 1000w PSU. Otherwise it's just not necessary and will cost you more to buy it. For dual 1080ti's however you might want to go with the 1000 just to be safe.
In fact, as far as I'm aware, since the 10 series cards are power limited I can even put a 1080ti in my system and my 600w will do just fine. So you still don't really need more than 850w bronze unless you are going for power efficiency to the max at which point like I already said you can go with a 1000w gold or platinum PSU.
In case you have decided to go with the 850w option here is my recommendation.
https://www.bhphotovideo.com/c/product/1349723-REG/seasonic_electronics_ssr_850fx_focus_plus_850w_80.html
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