Many text renderers allow the user to supply a suffix string and to use the suffix to supersede the assigned display units. This would be the way to get something like kWh.
In the units property, you can manually enter units which are combinations of other units, and you can use SI scaling prefixes as well. So kW, mm, and cm would all work without issue. For kWh, you can write this in the units, but it will be written / appear as
kW*h which is not ideal for display. Therefore for that unit, and the relative humidity, you probably want to use the text renderer suffix.
It is possible to add units to the units set, but there's not an especially clean way to go about it. I mentioned the UnitUtil class in these threads: https://www.google.com/search?q=site%3A%2F%2Fforum.infiniteautomation.com+unitutil