Power strip and power measurement method

A power measurement method comprising:

• applying a power supply voltage supplied to a power strip to both ends of a first light emitting diode included in a first photocoupler;
• monitoring a first output signal of the first photocoupler, a level of the first output signal being changed when the power supply voltage exceeds a first threshold, and thereby obtaining a length of a period when the level of the first output signal is changed;
• applying the power supply voltage, whose polarity is reversed from polarity in the first light emitting diode, to both ends of a second light emitting diode included in a second photocoupler;
• monitoring a second output signal of the second photocoupler, a level of the second output signal being changed when the power supply voltage exceeds a second threshold, and thereby obtaining a length of a period when both the first output signal and the second output signal are at a low level;
• estimating instantaneous values of the power supply voltage based on only the length of the period when both the first output signal and the second output signal are at the low level;
• measuring instantaneous values of a current for each of a plurality of jack portions included in the power strip, the current being supplied to an external electrical device from the jack portion;
• and
• calculating a power value of each of the plurality of jack portions individually by using the instantaneous values of the power supply voltage and the instantaneous values of the current,
• wherein, in the estimating the instantaneous values of the power supply voltage, an effective voltage value Vrms of the power supply voltage is calculated by using Vrms=aX+b (a is a constant having a unit of voltage per time and determined in advance, and b is a constant having a unit of voltage and determined in advance) where X denotes the length of the period when both the first output signal and the second output signal are at the low level, and
• the instantaneous value of the power supply voltage at a time point t is estimated as √{square root over (2)}Vrmssin(ω is a known angular frequency).