eMon Energy Monitor is one of the few Energy Monitors that measures True Power in Watts. Most only measure Apparent Power, i.e. simply multiplying a fixed 220V with measured Amps. That is only an indication of Watts consumed.

ACs, pumps, refrigerators, anything with motors have True Power that is around 20% less than simply multiplying Volts with Amps.

eMon Energy Monitor also measures Volt. In Indonesia it may be 220V in the morning and 200V by the afternoon. Again, most other energy monitors cannot do that. They only use a country’s reference Voltage. To accurately measure your Real Electricity Usage your device needs to measure Ampere, Voltage and Phase angle. eMon Energy Monitor does that every few milliseconds

A textbook explanation:

Chapter 11 - Power Factor

##### Reactive Power

We know that reactive loads such as inductors and capacitors dissipate zero power, yet the fact that they drop voltage and draw current gives the deceptive impression that they actually do dissipate power. This “phantom power” is called reactive power, and it is measured in a unit called Volt-Amps-Reactive (VAR), rather than watts. The mathematical symbol for reactive power is (unfortunately) the capital letter Q.

##### True Power

The actual amount of power being used, or dissipated, in a circuit is called true power, and it is measured in watts (symbolized by the capital letter P, as always).

##### Apparent Power

The combination of reactive power and true power is called apparent power, and it is the product of a circuit’s voltage and current, without reference to phase angle. Apparent power is measured in the unit of Volt-Amps (VA) and is symbolized by the capital letter S. Calculating for Reactive, True, or Apparent Power

As a rule, true power is a function of a circuit’s dissipative elements, usually resistances ®. Reactive power is a function of a circuit’s reactance (X). Apparent power is a function of a circuit’s total impedance (Z). Since we’re dealing with scalar quantities for power calculation, any complex starting quantities such as voltage, current, and impedance must be represented by their polar magnitudes, not by real or imaginary rectangular components. For instance, if I’m calculating true power from current and resistance, I must use the polar magnitude for current, and not merely the “real” or “imaginary” portion of the current. If I’m calculating apparent power from voltage and impedance, both of these formerly complex quantities must be reduced to their polar magnitudes for the scalar arithmetic.