Yes, it is neat. It provides a good insight into your power requirements, if not a perfect analysis. I too have seen some really strange failure modes in PCs due to an unreliable power supply. Very hard to diagnose. This tool helps select a power supply that will last the required time, and allow for component expansion or changeover. Thank goodness that CPU power requirement have at least plateaued, if not dropped, although GPUs are still hungry little buggers.
You must have some bits in that PC that I didn't guess right! The tool does use maximum power for each component I believe, but then uses the CPU and System "loading" levels you set to come up with a result. If you set everything to 100% you will get a much higher number. I typically look at 100%, but use the recommended 90% loading. I like that it includes an ageing allowance in the capacitor age, which you can also use to allow for 24/7 operation. See note 4.
I knew that our power was actually 230 volts, even though we still call it 240 volts. I didn't know about the power company exemption. Must have been to allow time for equipment lie cycle driven change over. I would hope that exemption expires at some time.
I reckon everyone building a PC should know about this tool.
(No, I have no affiliation. Just a happy user.)