I'm trying to get my head around how to use the number read back from reactive energy registers.
If your load is purely inductive, or purely capacitive, then I can see what you'd read would be a true indication of reactive energy over that accumulation period. But what happens when your load is a circuit with some horrible mix of inductors and switchmode power supplies? At any instant, depending on the state of the various devices on the circuit, it may appear as an inductive load or as a capacitive load.
Within a given accumulation period, if your load flips from being inductive to capacitive (or vice-versa) then you'll effectively lose some reactive energy, right? The reactive energy register would be getting larger during the inductive stage and then start getting smaller during the capacitive stage. Once that happens, is the actual number you read back from that register really useful for much? I guess it's useful as a long-term average of the "inductiveness Vs capacitiveness" of your load, but presumably it can't be used to calculate power factor for example, because of the "lost reactive energy" that occurs as they start cancelling each other out?