So if I have a 850 watt power supply does that mean I need a 1700 watt UPS
No. That's a rather "intense" UPS.
I can buy a 1600W power supply to run a pentium CPU and no GPU. That system will draw like, 100W under load. What you need to do is figure out how much power your system draws under load. Which is almost never the same as your PSU's rated maximum wattage. You can get close to that number by adding the TDP of your CPU and GPU(s), and maybe 50-100W extra for "everything else". If you overclock, then more, etc. But hopefully you understand a bit more about power draw once you start doing things like that.
Once you have that number, again, make sure the PSU is rated to supply that much wattage under battery operation. Then, pick a VA rating around 1.5x the power draw you came up with for your system.