Portfolio Manager doesn’t have the ability to track the energy you put into a “bank.” When you update this meter, you are asked how much was used onsite and how much was exported offsite. First, you should enter the amount of kWh that you generated and used at your building ("Energy Used On Site"). Then, you should enter the energy that you are putting in the “bank” in the exported column.
You don't get an energy credit for the energy that you export, but if you own the RECs, then you will get an emissions credit. In a following month if you use some energy credit from the “bank,” you enter it as an electric grid purchase. While your utility may give you a “credit” for this energy, the actual energy that you are using comes from the grid, not from your solar panel.
The figure below illustrates that there are three flows of energy: energy generated by your system and used at your building (B); energy generated by your system and exported to the grid (A); and energy imported from the grid to your property (C). Each month you will likely have all three flows of energy at your building (and almost always B & C) because at any given moment you are either using your solar power or exporting it. When you look over the course of an entire month there can be time periods for each. Therefore, even when you get a negative “net” bill from your utility, that doesn’t mean you didn’t have an imports from the grid, it just means you exported more than your imported.