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hi

i'm shobha. i referred to the below link and have done a circuit which measures live electricity

http://openenergymonitor.org/emon/buildingblocks/how-to-build-an-arduino-energy-monitor?page=15

but i think the readings which i'm getting in my serial monitor are not the right ones. i tested first for a 60watt incandescent bulb but the readings in my serial monitor varies continuously and it shows 52watts. since the incadescent bulb is purely a resistive load as per my knowledge there should be no losses and i have to get 60watt in my serial monitor.

i think my calibrated values are not right,  i asked one of my friend who is a electrical engineer he says the voltage everywhere is not constant and it fluctuates . so my doubt is how i need to calibrate ? do i need to calibrate everyday?