Hi Dana 
I don’t know what the HEM defaults to (I think 120v but not certain) however you can change the ‘assumed voltage’ which the HEM uses for it’s energy calculations. This is specific to v1 as I believe the HEM v2 actually measures the line voltage directly.
When looking at the code on github, line 785 (as of this post) the following configuration value will be found:
zwave.configurationV1.configurationSet(parameterNumber: 1, size: 2, scaledConfigurationValue: 240).format(), // assumed voltage
If you change the ‘240’ to ‘120’ (or whatever your actual voltage is) then it should work correctly. However when I was on this learning adventure I do remember the HEM being cranky with it’s settings. So, be sure and validate the calculated wattage with another device to make sure the HEM is accepting the updated voltage setting.
Edit: I’ve updated the main post notating the option for 120v versus 240v.