Hmmm... this is a month old with no replies - I am surprised. I am not a Mango expert, but what you want is what Mango is all about so I will try to point you in the right direction.
Once you have the Modbus up and running, you need to create and configure one or more Data Sources. There is an icon for this. Once this is done, start adding and configuring data points for each data source.
To calculate total energy you can define a Meta Data Source, and data points in it can be set up to be calculated from raw data from your PV system.
This is all basic stuff for Mango and there should be items in the forum that will answer your questions (and maybe already have?) ;-)