Thank you, @Robert.Wall, for connecting me with the appropriate schematic. From this I conclude that an isolation transformer is not required.
The next step was to determine the Watt spec for the 10k resistors. Granted, we can use the RMS voltage. 220V/60k = 3.6mA. 10kΩ * (3.6mA *3.36mA) = 0.129W which means 1/4W resisters will be just fine.
To better understand, I do further calculations. This current will be on the sececondary and with a 75Ω resistor, The secondary voltage will be 0.275V. From the emon schematic we see that the offset is 3.3V*33kΩ/(33kΩ+180kΩ) = 0.511V which is more than lowest negative voltage.
The offset plus the secondary voltage (0.786V) needs to be less than Vref. This come from the MCP1501-10E/SN which, if I read the specs right, has an output of 1.0240V, The schematic shows that the output is voltage divided by two 10kΩ resistors for an output of 0.512V because AGND is tied to GND. The voltage divider doesn’t seem right?
But then I thought about VRMS. 220VAC RMS is 622Vp-p. Plugging in 622 to the above equations gives us 10.5mA and a secondary voltage of 0.778V which is greater than the offset. That means, for a short period of time, ADC has a negative current. Not good on the positive peak, either, because 0.778V + 0.511V = 1.289V which is more than Vref.
Obviously this all works. But why?
I am uncertain, but I believe that the Arduino has a diode that would be forward biased on the negative swing and will disapate this energy which might explain why the Arduino doesn’t blow up. I don’t know what happens when Aref is exceeded.
Wouldn’t overvoltage affect the energy calcualtion because the wavefore is clipped on the positive and negative swings? (I haven’t looked at the software but I assume that it alternates sampling the voltage and current waveforms over one cycle (~80 samples each) and then calculates the power after all of the samples are taken.)
I am hoping with your guidance I can better understand what is going on here. I sincerely appreciate your thoughts. Thank you.