xSPOONx

A Kill-A-Watt or whatever other power monitoring device should be accurate. You need to know both the Voltage and Amperage. You can also get whole home monitors that can monitor the entire feed or individual circuits.

It should play nice with x264, but it won't touch x265 as that requires Kepler or newer. You'd be better off selling it and getting an Intel A310. Half the power usage while also having far, far more encoding power. It could also be used as a decoration.

As @Magnus said, very difficult, without an external reading from a meter or "smart" PSU like Corsair had for a time. Without knowing what efficiency the PSU is running at, the CPU pulling 40W, the GPU pulling 30W, and the motherboard and drives doing another 30W is meaningless without knowing the efficiency of the PSU - which is going to be dynamically changing with the load. It https://www.tomshardware.com/reviews/corsair-rm550x-power-supply,4484-5.html https://www.tomshardware.com/reviews/fractal-design-ion-560p-power-supply-review,6279-2.html Below 50W even 550(ish)W PSUs fall on its face. So, that 50W indicated is more like 100W at the wall. There will be significant variation here because the PSUs efficiency is so important. This example is double. If you're constantly pulling 100W, you're around 88%-91% efficient, so you're pulling ~110W-114W at the wall depending on the PSU. A PSU that's only 80% efficient at 100W will pull closer to 125W. So, knowing what your components are doing only tells you part of the puzzle. There's a significant unknown variable at play that you need to account for if you want accurate readings as to what's happening.

Check out the Intel A310.