The main objective of this study is to evaluate the impact of building energy efficiency measures on the cost of solar electricity in a climate-compatible villa located in the suburbs of Saman, Chaharmahal and Bakhtiari Province, Iran. Enhancing building energy efficiency while lowering the cost of renewable electricity generation is particularly vital in Iran’s off-grid residential sector, where growing energy demand and dependence on fossil fuels necessitate sustainable, climate-compatible solutions. A baseline case and five optimization scenarios were modeled using DesignBuilder (v6.1.0.6) to estimate annual energy consumption, followed by techno–economic–environmental assessment of an off-grid solar–battery–diesel generator system using HOMER (v2.81). Results show that the net present cost (NPC) of the baseline system is $947,243, with cost reductions of 17.6%, 5.4%, 22.7%, 63.1%, and 79.5% achieved through polystyrene insulation, green roof, UPVC windows, VRF HVAC, and all measures combined, respectively. The optimal integrated scenario also reduces annual emissions by ~130 tons and increases the return on investment (ROI) by 146%. This work uniquely couples building-level energy efficiency modeling with techno–economic–environmental optimization of a hybrid off-grid PV– Battery–Diesel generator system, quantifying for the first time how demand-side measures propagate into key renewable electricity cost metrics in off-grid residential contexts. These findings highlight the substantial economic and environmental benefits of combining building optimization strategies with renewable energy deployment in off-grid residential applications.