AC, Negative instantaneous power x Wh consumption measurement
How this &quot;negative power&quot; affect the Wh real power consumption
measurement? How it accounts in cumulative energy measurement?

Here are a few waveforms to consider: -

The power waveform is in red and, it is the average value of this waveform that indicates true power consumption. If the instantaneous power goes negative then it just reduces the average power. At 90 ° phase shift between voltage and current, the average power is zero because positive parts exactly cancel negative parts.