Multi-variable least-cost optimisation of residential efficiency retrofits

• GA tool optimizes envelope, HVAC, DHW, PV, and batteries together. • Moderate envelope upgrades are cost efficient. • Heat pumps with hybrid PV battery outperform gas systems. • High-star upgrades existing stock do not achieve cost-optimality under assumed conditions. • Tool scales from single dwellings to portfolio studies. Existing residential retrofit tools typically optimise a narrow set of measures, use simplified economic assumptions, or are not directly compatible with industry-standard energy rating software, limiting their usefulness for practitioners and policymakers. This paper presents a Lifetime Energy Cost Model (LECM) that couples AccuRate building energy simulations with a tariff-based energy cost model and uses a genetic algorithm to identify least-cost energy efficiency retrofit configurations for existing dwellings. The model minimises 15-year annualised cost under current and projected energy tariffs, retrofit installation costs and rebates. The framework simultaneously explores building envelope retrofits (floor, wall and ceiling insulation, and windows), electrification of HVAC and water heating appliances, rooftop solar and battery storage. The LECM is applied to a representative pre-2000 detached dwelling in Melbourne. Validation against a brute-force search for restricted design spaces confirms that the genetic algorithm reliably identifies the global minimum, demonstrating robust convergence and computational efficiency. For the case-study dwelling, the least-cost configurations prioritise cost-effective envelope upgrades involving R2.0 ceiling insulation, R2.0 floor insulation, R1.9 blow-in wall insulation and improved seals for airtightness (raising the NatHERS energy rating to around 5 stars), replacement of gas appliances with heat pump HVAC and hot water systems, and the installation of a rooftop solar system of around 4.0 kW. Together, these measures reduce energy bills by approximately 75% with a payback period of about 9 years. Larger export-oriented solar systems and battery storage are only financially attractive under specific rebate and tariff conditions. Sensitivity analysis indicates that capital cost and financing assumptions are the dominant drivers of economic viability, with energy tariff forecasts exerting a secondary influence. The results illustrate how an integrated, cost-focused optimisation framework can support the design of practical, least-cost decarbonisation pathways for existing housing in temperate climates.