Power Factor sign is subject to standard used, that is either the IEEE or IEC standards.
A comparison of the 2 standards is presented:
https://www.se.com/ww/en/faqs/FA212521/
PF sign convention: IEC
PF sign correlates with the direction of real power (kW) flow.
- Quadrant 1 and 4: Positive real power (+kW), the PF sign is positive (+).
- Quadrant 2 and 3: Negative real power (-kW), the PF sign is negative (-).
PF sign convention: IEEE
PF sign is correlates with the PF lead/lag convention, in other words, the effective load type (inductive or capacitive):
- For a capacitive load (PF leading, quadrant 2 and 4), the PF sign is positive (+).
- For an inductive load (PF lagging, quadrant 1 and 3), the PF sign is negative (-).
PF sign test on Shelly EM:
I test the sign of PF given by the Shelly EM on the 4 quadrant by looking the “power”, “reactive power” and “pf” (publish with mqtt) and obtain:
- For a capacitive load (PF leading, quadrant 2 and 4), the PF sign is negative (-).
- For an inductive load (PF lagging, quadrant 1 and 3), the PF sign is positive (+).
It seems that Shelly EM use (IEEE PF sign convention) multiply by (-1)