Peak demand based billing is only applied to large commercial and industrial supplies here. Thankfully.
I think we have different aims. If the energy monitor I have tells me I'm drawing 450W and that figure it out by 10% because one of the loads is inductive like the washing machine, I'm fine with that.
I do not need this to verify or cross check the electric utility meter (magnetic rotating disc and clockwork digits type). I really just want indicative, comparable measurements.
If logged and graphed this can show me an estimate of the base load and see where and how hard the spikes hit when the "big hitters" are used, shower, washing machine, cooker, space heaters etc.
As to hacking into an off the shelf module, it's took fiddly. As to the USB style monitors; they often save a CSV file to a memory card and so not realtime data, that or a prepriatory file which you have to pen with their "app".
I might just follow the open energy monitor spec and see how far I get.