This paper presents a comprehensive techno-economic calculator for evaluating residential photovoltaic (PV) system profitability in Croatia under two distinct regulatory frameworks: the legacy monthly net-metering scheme and the new 15-minute interval net-billing scheme mandated by the 2025 amendments to the Act on Renewable Energy Sources and High-Efficiency Cogeneration. The transition from monthly aggregation to sub-hourly settlement fundamentally alters the economic value of self-generated solar energy by requiring near instantaneous matching of generation and consumption. Using high-resolution time-series modeling coupled with standard financial metrics —Net Present Value (NPV) and payback period— we quantify the profitability impact across realistic residential load profiles and Croatian-specific tariff structures. Results demonstrate that 15-minute settlement reduces NPV and extends payback periods compared to monthly net-metering, with the magnitude depending on self-consumption ratios, system sizing, and tariff parameters. For a representative 4.5 kW residential installation, the payback period increases from approximately 6 years under monthly net-metering to 12 years under 15-minute net-billing. The calculator provides transparent, reproducible decision support for Croatian prosumers navigating this regulatory transition and contributes methodological insights applicable to similar policy shifts across European Union member states.