This paper deals with the thermal analysis and the auxiliary electrical energy saving in a passive-solar-heated residence. The particular residence is using an attached sunspace of inclined walls and roof and an electrically heated thermal storage floor as backup to satisfy the heating requirements, The thermal analysis and the corresponding electrical energy saving are determined using three different methods which are applied for thermostatically controlled temperature regime. The first is a simplified method based on three parameters: (1) the solar/ load ratio (SLR), (2) the building thermal inertia and (3) the monthly unutilizability (UU). The other methods are, namely: the UU method and the SLR method.