Why Transformer thermal loss ignore no-load loss when generator is OFF

edit

I have to simulate a PV generator connected to the grid by a transformer, and I need to evaluate thermal loss of transnsformer, but there is a problem. Transformer thermal loss ignores transformer no-load losses. Although I have set the availability of the transformer and the PV generators to "always 1" and the no-load losses of transformer are different from zero. The no-load losses are correctly added when the photovoltaic generator produces energy, but during the night (that is, when the photovoltaic is not working) the thermal loss of the transformer is zero. Can anyone suggest me how to solve the problem? Thank you.