MontyMc
I currently own 2x Gtx 1080s and I wanted to upgrade to the ti and my current psu is only 850w. Can someone help me out.

 
It would be really great if you gave more details about your system. Also knowing the brand of your PSU would help. A quality brand 850 watt is a maybe depending upon the rest of your system.

I can tell you from personal experience that my computer draws around 830 Watts under full benchmarking load.  See signature for specs.  This is a rather rudimentary measurement using the percentage load on my 1000 Watt APC UPS which, as mentioned, will go up to around 83% load.  If you have a high quality PSU, the 850 might be enough for you, but for piece of mind, get the best PSU you can afford.  Also keep in mind that I have 11 fans, and 2 Hybrid kits installed along with a H115i AIO, so there's 3 pumps in there working and drawing power as well, so your load may be less than mine.
 
Lastly, if you plan on overclocking, you also need more power headroom.  My cards are founders editions with EVGA Hybrid Kits and during benchmark runs, I am running +200 core and +500 memory.  The CPU is a i7 6850k which is running at 4.3 Ghz using adaptive voltage set to a max of 1.325 V, all that stuff adds up and draws more power.

Always a good idea to have more watts versus required so Personally? I would say it would be a very good idea to upgrade.

Then again, I made the upgrade myself when I went from a XFX BES Black Edition 850W when I was running 2 970's to a EVGA 1300 G2 1300W PSU. Your PSU degrades over time, so your really not leaving much longevity with a 850W PSU and that's even if its a great model PSU.

Just my 0.02 somewhere at some time you're gonna have to upgrade or replace.

Yes, more details about your system would be nice, as HeavyHemi said, but you can use the OuterVision Power Supply Calculator (as linked to by JonnyGURU) to get a good estimate of your overall system wattage requirement and a recommended PSU wattage..There's good info in the PSU FAQ section at JonnyGURU..An 'oversized' (wattage-wise) PSU can be desirable.

You can also use the EVGA Power Meter that will recommend PSUs based on your system hardware..You will recieve a 10% discount code in your email that can be applied toward the purchase of any EVGA PSU, should you need one, just simply for using the Power Meter..The 10% discount code is only good for 30 days and can only be used in the U.S. region, last I knew.

Yeah I would upgrade from PSU to be safe, I mean it can work, but also not depending on how old it is, which could lead to random system shutdowns, let alone the faster degrade.
And when the PSU is working at almost full capacity the ripple often increases, this means a less stable electrical signal and may also cause stability issues or even faster degrading of components if it's very bad. (bad for overclocking stability ofcourse too)
 
Aside of that the PSU wouldn't get so hot if you have a A-brand 1000W or higher PSU, the best efficiency is usually around 50% load.
In my experience my AX1200i actually barely kicks in the fans because it's topping out at around 600w with a single 1080Ti with the specs in my signature below, no overclock and 117% power target.
 
Here's some readings from the PSU with GTA V for 52 minutes with an ambient of about 24°C:

Sure he is the details of my pc.

Intel Core I7 6800k
Asustek Strip x99 motorboats
Corsair 850w PSU

No need to upgrade from an 850w. 850w has been recommended for two high-end sli cards for years now. A 1k watt psu will work too, but it's just wasted money. I prefer Evga and Corsair psu's personally. I've used an HX850 on two 280's, 580's, 680's, 780's, and now my 1080ti. Still kicking. My psu is rated at 850w output (not from the wall) continuous at 50c, I believe. Outside of a specific psu stress test, my system never saw more than 500ish watts pulled from the wall with one of those meter thingy mabobs.

850 W will work fine as long as you are not hammering your CPU and both GPUs at the same time.
 
I ran an 850 W PSU for 2 Titan X Pascal cards.  No issues.
 
As this system runs folding @ home 24/7, I eventually upgraded to a 1050 W Titanium (EVGA of course) for the increased AC to DC efficiency (i.e. lower electric bill and less heat from the PSU.)  This system typically draws 825 W from the wall when folding (no CPU folding, just GPU folding).  You multiply that by the efficiency number (at that power draw) to get the actual power draw by the system.

No, even when hitting the CPU and both GPU's at the same time, ie gaming, he will be fine. Even running OCCT's psu test, my system below maxed out at 800w from the wall. With an 80% effeciency (at best) this means the system is only pulling ~640w, and at probably 70F room temp, much lower than the 50c it is rated. Also keep in mind the psu is rated at 850 OUTPUT, not 850 from the wall.
 
Below is my current system specs but I've had SLI cards up until my 1080ti, but through the years power draw has been in the same neighborhood. In no way during even gaming did my system hit 800w draw from the wall. This was only done during specific stress tests that did everything from spinning the hard drives, linpack the cpu, rendering on the gpu's, etc. Realistically it was more like 400-500 max from the wall even with two high-end cards. I'll be glad to retest with a Kill-A-Watt.
 
4790k @ 4.6ghz, Thermalright Silver Arrow, 16gb Gskill ddr3 1866, GTX 1080 Ti, Acer XG270HU, Corsair HX850, Gigabyte Z97X-UD5H, Samsung 850 Evo 250gb, WD Black 1.5TB, WD 2TB External, WD 4TB External, Klipsch Promedia 2.1, Silverstone RV02-E, Logitech G500, Nuforce HDP & Sennheiser HD650
 
Edit: It looks like I need to update my signature.

In most scenarios you should be just fine, just make sure it has the proper connectors to power the thing

One of my Folding rigs is 2 x 1080ti on a Seasonic X-850 gold.
Uses a bit less than 600 watts peak at the wall, but I don't have it overclocked hardly at all, TDP is set to 100% on a pair of 250 watt TDP 1080 ti cards, with a pretty minimal system otherwise (G4600, 8 Gig RAM, HGST hard drive).
 
 If you don't have a TON of other system stuff and don't overclock, a good 850 should be enough - but if you do have a lot of other stuff, like high-flow fans or water cooling and high-TDP 1080 ti varients or want to push the TDP for serious overclocking, I'd move to a GOOD 1000 watt supply as a minimum (Corsair varies, some are good some are *meh* depending on who made the specific model for them).
 

Nec_V20
The simple answer to this it no.
 
If you have a GOOD PSU and by good I mean something such as the Corsair AX860 which I use then you can easily power a dual 1080 Ti setup.
 
I think that some might be mistaking what they draw out of the socket instead of what the PSU is actually delivering.
 
For instance, if you have an 850 Watt Platinum rated PSU then, if you are drawing 870 watts AC out of the socket, the amount of DC power being supplied to your system (assuming an 88% efficiency 110V) would be (88 / 100 * 870) 765.6 Watts which would be well within the specs of an 850 Watt PSU.

+1
 
240v in UK but above quote is right enough 

baconinabun

Nec_V20
The simple answer to this it no.
 
If you have a GOOD PSU and by good I mean something such as the Corsair AX860 which I use then you can easily power a dual 1080 Ti setup.
 
I think that some might be mistaking what they draw out of the socket instead of what the PSU is actually delivering.
 
For instance, if you have an 850 Watt Platinum rated PSU then, if you are drawing 870 watts AC out of the socket, the amount of DC power being supplied to your system (assuming an 88% efficiency 110V) would be (88 / 100 * 870) 765.6 Watts which would be well within the specs of an 850 Watt PSU.

+1
 
240v in UK but above quote is right enough 

You might have noticed that although I said Platinum I only put the efficiency at 88%. If you use 240V as we have in the UK then the PSU will be about 2% more efficient. So in the example I gave above, if you were drawing 870 Watts out of the socket then the amount you would be supplying to the hardware would be (90 / 100 * 870) 783 Watts.
 
One thing that I forgot to add, is that if you are running your system off the 850 Watt PSU and you are just doing normal stuff like browsing the Internet or watching videos then the PSU will be a lot closer to its "sweet spot" where it is most efficient (50% of the rated Wattage +/- 10%) than the 1000 Watt PSU. This means that the 850 Watt PSU would save you money on electricity when you are just faffing around and not gaming compared to the 1000 Watt PSU.
 
BTW I grew up in Cathcart, about half a mile down the road from Hampden Park in Mt. Florida