I have a very simple configuration of a load, PV, and the grid. With this configuration I obtain a yearly grid purchase of 2384 kWh/year. If I remove the grid, I expect to obtain an unmet electrical load equal to the yearly grid purchase of the previous case but instead I obtain an unmet electrical load of 2082,5 kWh/year. This doesn't make sense -- could you explain to me where the difference comes from?

The converter in this model has the "Parallel with AC generator?" option unchecked. HOMER actually considers the grid to be a "generator" when it is purchasing power from it, and so the converter can't operate in any time step that the system is purchasing power from the grid. So, in any time step where the PV does not meet the required operating capacity on its own, the energy is instead purchased from the grid and the converter is "off" (so the PV output is wasted). So, if you check this box, the model will behave as you expect.

You should upgrade to the latest version of HOMER Pro here: https://users.homerenergy.com/homer-pro/download

In addition to a number of bug fixes and improvements, the new version (currently version 3.2.3) display a warning when this option is unchecked. Many users have been caught unaware by this feature, so we added the warning. The "Parallel with AC generator?" option is necessary to model certain systems, since some converters can't sync with the grid, and so they must turn off any time the grid is used.