Hope I can help,
i currently do this using influx db, this is my singlestat for Exported Power:
This is the calculation:
My export tarriff is 11.3 cents (0.113 in the calculation)
On the options tab, set the STAT to total:
This works for me and calculated the same as PVoutput.
The setting in iotawatt for output to influx db is below:
I have 2 single stats showing imported and exported based on the above outputs from iotawatt.
Hope this helps.