How is battery depletion calculated?

The battery depletion is the difference between the battery's state of charge (SOC) at the start of the year and the SOC at the end of the year.  The SOC at the start of the year is specified in the battery component window under the title "Initial state of charge (%)".

The difference occurs because HOMER simulates the system for a year and does not require that the SOC is the same at the start and end of the year.