What is the simplest and most cost-effective device for calculating power usage?

[jpgnfx]

Hi,

can anyone help with advice on what basic device can i use to calulate power usage?
the implementation I am looking at is follows

mains socket -- device -- appliance

its much like the product -> http://www.p3international.com/products/special/P4400/P4400-CE.html except i want to do away with the LCD and strip it to the bare minimum to basically reduce cost..

is there a basic device i can use? if I am not wrong, the device measures current and uses P = VI to convert to wattage.

thanks in advance

 

[anorlunda]

if I am not wrong, the device measures current and uses P = VI to convert to wattage.

Not quite that simple. P=VI works only with unity power factor. That may or may not be a good approximation for you depending on the load. If the load is a motor or a transformer, the power factor will be lower. If it is something like an electric stove, power factor will be close to 1.

There are devices that do all the needed calculations for you. For example here
https://www.analog.com/en/products/monitor-control-protection/power-monitors.html

But building your own meter is a big project.

Can you just buy a meter that you plug the appliance into, such as: https://www.amazon.com/dp/B00009MDBU/?tag=pfamazon01-20
They cost less than $20.