Could you explain me how the excess electricity is calculated in homer and what does it mean?

In HOMER, excess electricity occurs whenever the system produces more electricity than the system can use.  It will occur whenever the power production exceeds the load and the batteries or grid can’t absorb all the surplus.  But it also may occur due to insufficient AC/DC conversion capability.  For example, if a like a PV array on the DC bus supplies an AC load, but the PV array is producing more power than the inverter can handle, and the system does not include a battery bank or the battery bank is full, excess electricity may occur on the DC bus.