It sounds like Larry is missing the concept of scale here.
Larry,
What Robert and Paul are trying to say is the voltage supplied by the AC to AC transformer (wall-wart) is lower than, but proportional to, your 240 Volt AC Mains Voltage.
The voltage sample from the transformer is used to determine the phase relationship between voltage and current to enable the hardware to calculate real power, i.e. what you get billed for. Without the transformer, the hardware uses a fixed value in the sketch to make the calculation. The result of that calculation is only an estimate of apparent power. We want an accurate measure of real power.
Robertâs graph shows how the voltage at his location varies througout the day. That happens everywhere
and is one of the reasons the AC wall-wart is needed for accurate measurements. i.e mains voltage doesnât stay at one value. It varies, over a fairly small range, constantly.
It doesnât âknowâ anything. The onus there is on you. If you do as PB66 mentioned two posts above this one, itâll work fine. I ran an emonTx for about two years with a 120 Volt wall-wart as an AC voltage sample source, and it worked like a champ. The emon part of an emonPi is essentially a stripped down emonTx.
To get optimum accuracy, youâll need to perform the calibration procedure.