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
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.
How does the unit calculate kW?
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.