This is what you use to calculate power draw:
Just plug in the numbers and you'll get the power draw of the CPU or GPU you're overclocking once it's overclocked. It does have a few issues it only works for specific temperatures and TDP doesn't always give accurate values it's best if you measure the initial power draw rather than relying on the TPD to be accurate. However if you do you power draw and stock voltage measurement when under load and your temperature when overclocked isn't different this will be accurate. A power draw value isn't all that useful when you're trying not to kill a VRM. To get current draw you just use this version of it:
The power draw value is good for when you're designing a chiller or trying to choose coolers the current draw value is for not killing a VRM. The equation has a +/- 10% margin of error so keep that in mind when using it(AFAIK it does not work with Nvidia Maxwell GPUs at all). It can be much more accurate if I could do more research into it but that requires that I can get my hands on a bunch of CPUs to test it with and several liters of LN2 so I can keep very specific temperatures.
Just for the sake of doing a quick proof:
My 3960X measured 353W at 5Ghz using 1.56V running IBT. It has a stock voltage of 1.18V, TDP of 130W and stock clock of 3.3Ghz. Lets plug them in:
So it a little bit of but hey better than nothing.