Question 1

I understand most figures but what is "power factor" ?

Accepted Answer

As degree qualified electronics engineer, I can say most of the answers here are rubbish. Although, there are several definitions if you look up on the internet, but I will write here the exact mathematical definition for it.

Power factor is a measure of the phase angle difference between the voltage and current.
Concepts of real and apparant power come into it.
Real power is a measure of the power where the voltage and current are in phase. Apparant power is a measure of the power when the voltage and current are not in phase.

It's to do with resitive and inductive/capacitive loads. Inductive loads are devices which use magnetic fields in their operation such as electrical motors and even heating elements and electrical irons which contain heating elements implemented as coils of wire. A purely resistive load results in the voltage and current being in phase with each other. It is capacitive and inductive loads the which shift the phase of the current relative to the voltage, either forwards or backwards.

The mathematical definition is:

P=V.I.Cos(theta)
P is the power
V is the voltage
I is the current.
Theta is the phase angle difference between the voltage and current waveforms.
The full stop symbols in this equation means "multiply".

Cos(theta) is the Power Factor. And the Cosine of any angle is a value between 0 and 1.
A value of 1 represents the voltage and current waveforms are in perfectly in phase with each other, where the phase difference is zero degrees, and therefore the cosine(0 degrees) is the value of 1.
 
A power factor not equal to 1, means there is apparant power being consumed.
As residential customer you don't need to worry about it. You don't get charged for the apparant power (not yet anyway although that might change in years to come with smart meters). You only get charged for real power.

The maths involved and true understanding is university degree level, though they will teach the basics to electricians. Don't worry about it. You can ignore it in a residential setting.

Question 2

Will it display as low as say 0.2 watt accuratly ?

Accepted Answer

Yes, the Modes 2 cumulative electrical quantity smallest energy value on the display is 0.001Kwh.

Question 3

Is there a way to zero the data (kWh, Lo watts, hi watts) without resetting the entire unit and then having to re-enter the cost per kWh?

Accepted Answer

Hold Function and Up. That resets the accumulated data but doesn't reset the cost per kwh.

Question 4

I'd like to monitor the voltage in a socket for a period of time (24h and a week). Can I do it with this product? How does the memory function work?

Accepted Answer

It has day functions but I’ve never needed to record the data to that extent.

ASIN	B085S7Q1T4
Best Sellers Rank	25,828 in DIY & Tools ( See Top 100 in DIY & Tools ) 8 in Power Monitors
Brand	MECHEER
Colour	black
Compatible Devices	Any device that can be plugged into a standard 3-pin UK socket
Country Of Origin	China
Customer Reviews	4.3 4.3 out of 5 stars (2,532)
Included Components	1x Power Meter 1x User Manual 1x Reset Tool
Item Dimensions	7 x 7 x 16 centimetres
Manufacturer	MECHEER
Manufacturer Part Number	JK-PM01-UK
Maximum Operating Voltage	250 Volts
Measurement Type	Electrical
Minimum Operating Voltage	230 Volts (AC)
Model Number	JK-PM01-UK
Number of Items	1
Power Source	Corded Electric
Specification Met	EN 61010-1, UKCA
Style Name	Modern
UPC	743829258749
Unit Count	1.0 count

Power Meter Plug Energy Monitor Watt Backlight LCD Display Electricity Usage Consumption Analyzer Voltage Amps Wattage KWH Outlet Socket, Updated Version 180° Viewing Angle

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