Te current version of OpenEVSE does not have a sensor to measure voltage. The code was added for an experimental version built by Nsayer that he called OpenEVSE II.
How does the unit calculate kW?
Current in watts x 0.240 = Power
If your voltage under load is not exactly 240 you can change the multiplier to match your voltage.
Using the RAPI commands, a $GM is issued and $NK is returned. When a $GG is issued, $OK 0 -1 is returned.
According to the RAPI documentation, $GM is a valid command. With regards to $GG, it states if -1 is returned then the voltmeter must be defined but not sure what needs to be done to define it. Is it SM?
Using $GV, the version is 3.11.3 1.0.3