Due to the nature of the electrical load on the circuit — whether it be inductive, capacitive, or involving components such as motors or coils — it can significantly alter the power factor (PF) readings, often manifesting as negligible wattage. This is a case with digital power metering devices such as Shelly EM/3EM/Pro EM/Pro 3EM. In an example scenario, a -10W readings likely stems from the presence of inductance or capacitance within the circuit. Even when a load appears to be in an idle state (or switched off), residual capacitance can still register on the measuring device due to the sensitivity of current transformers (CTs) or as a result of electrical noise.
In such instances, it's advisable to employ a measuring device directly connected to the load, like the Shelly Pro 1PM, for more accurate readings and to bypass potential interference or false signals caused by the circuit's characteristics.