Sadly, a lot of people (even on these forums) keep recommending massively overpowered PSUs for no reason other than that it used to be smart back in the day. Heck, the entire >1000W PSU market (outside of mining, which is largely dead anyhow, and extreme LN2 overclocking) is held up by people buying stupidly overkill hardware. 750W is sufficient for most
dual GPU setups today, and the most common single GPU setups (say, a GTX 1660 or RTX 2060 with an SSD, an HDD, 16GB of RAM, some RGB and a reasonable CPU around 95W) would struggle to exceed 300W internal power draw under normal gaming usage. PC components these days are very, very power efficient, and due to the limitation of modern process technologies, they don't generally scale all that much when overclocked either. The days of 500W overclocked GPUs are long gone. There are of course exceptions, like the 9900K (which, to be frank, is a power hog even at "stock" on most motherboards), but those are the exception, not the norm. Also, it's worthwhile to remember that no normal workload stresses the CPU and GPU both at 100%. Gaming is generally 100% GPU and a medium CPU load. Rendering generally stresses either GPU or CPU 100%, but rarely both. And so on.
My rule for calculating the PSU requirements of a build go roughly like this: real world power CPU power draw from a review + real world GPU power draw from a review + 15-25W for the motherboard and RAM, 5W for each SSD, 10W for each 3.5" HDD + a couple of watts per fan + 5W for each AIO pump + whatever is needed for any other add-in-cards, components or power-hungry periperhals (usually not applicable). If overclocking, look at power draw numbers from someone overclocking the same component, and perhaps add 20% for safety/bad luck in the silicon lottery. Sum that all up, then add 20% to the total for some margin (as menitoned above, running your PSU for extended periods at 100% load will shorten its lifespan), and you're good to go. Not all these numbers reflect actual max power draw for each component (NVMe SSDs often peak at 5-7W, some fans draw 5W or more; 3.5" HDDs can peak at 15 or even 20W during spin-up), but they are more than sufficient for calculating the sum total power draw of the PC under normal usage.
So, for your proposed hardware (I'm assuming you mean the i7-9700K, as a 6700K can't run on a Z390 board):
- i7-9700K: TPU says 41W idle and 180W running Cinebench nT for the full system on a Z390 board, so ~140W. As this is far above TDP, this test result likely reflects MCE or similar unlocks, and is reliable. Overclocked to 5.1 GHz the 180W number jumps to 237W, so the CPU draw jumps to about 200W total. These full-system power measurements include PSU losses, so these numbers are perfectly safe to work with.
- (in case you actually mean the i7-6700K, but with a compatible motherboard: Hexus says 23W idle and 99W video encoding for the full system on a Z170 board, so ~80W stock, but this varies with motherboard power settings, so let's stick to the 91W TDP. Don't have overclocked numbers for this.)
- GTX 1080: Let's assume you're going for an overclocked model, so I'm basing my numbers on the TPU Zotac AMP! Extreme review, which measures out at 245W peak gaming, and 222W average gaming. Comparable numbers for the non-overclocked FE card are 166W and 184W, so this is pushing it for a 1080. Still, let's say 245W for safety, or 190W if running a lower end card at stock clocks.
- Cooling: if you're going for an AIO on your GPU, you'll likely have one on your CPU too, so 10W for pumps, and let's say 15W for four 120mm fans.
- Motherboard and RAM: 25W. This is a rough estimate, but unless your motherboard has something crazy like 10GbE networking, or you're running >4000MT/s RAM at 1.5V or more, it will do.
- RGB? I'm guessing yes, so another 10W for that.
- You didn't mention storage, so I'm assuming the relatively standard 1 SSD (5W) + 1 HDD (10W).
That leaves us with a total of either
486W (405W+20%, 9700K stock + 1080 stock), or
624W (520W+20%, 9700K OC + 1080 OC). In other words a 500 or 550W PSU would be perfectly fine for the former configuration, and a 650W one for the latter. If you meant the 6700K, subtract 60W from each number. Also note that even these numbers are overblown: adding up power draws like this
is not how power draw in PCs actually works. As said before, no normal consumer workload will stress your CPU
and GPU to 100%, so these numbers being based on max per-component power draw for the key components means significant built-in safety margins even before the added 20% - the 20% is for longevity, meaning your PSU can age to where it loses 20% of its output capacity (which will take many, many years for a good unit) and still handle your setup just fine. To add some perspective to this: TPU's 9700K review has full-system power measurements while gaming with a 1080 Ti, and they hit
340W stock or 376W when overclocked to 5.1 GHz. At the wall, including PSU losses. And that's with a significantly more power hungry GPU than the regular 1080.
I have two 650w's and two 850w models and they are brilliant units. I believe Scan or Ebuyer sales them at a decent price.. 
