Residential Photovoltaic-Battery Systems in Japan’s Post-Feed-in-Tariff Era: Economic Viability and Resilience Evidence from Kyushu

Japan’s residential energy transition is shaped by imported-fuel exposure, post-feed-in tariff (FIT) value erosion, tariff uncertainty, and disaster risk. This study evaluates whether a fixed 4 kW photovoltaic (PV) and 5 kWh battery energy storage system (BESS) package can create sufficient private household value over a 25-year lifecycle. The analysis uses audited 30 min electricity-demand profiles from four detached households in Kyushu and a broader 181-household demand context. A perfect-foresight mixed-integer linear programming model provides an upper-bound dispatch benchmark. Dispatch outputs are evaluated using net present value (NPV), retail-price escalation, time-of-use tariff sensitivity, value of lost load (VoLL)-based outage valuation, forecast-realism adjustment, and a third-party ownership (TPO) benchmark. PV-only investment remains profitable, with a mean NPV of JPY 338,007, whereas adding BESS lowers mean household NPV to JPY −907,448. The implied break-even BESS capital cost is JPY 39,840/kWh, far below the JPY 200,000/kWh base assumption. A +50% retail-price escalation case improves mean PV-BESS NPV by JPY 788,381 but remains below break-even. During a 72 h outage, BESS serves 84.8% of assumed critical demand. The limiting factor is insufficient monetizable household value, not technical usefulness.