Home energy efficiency upgrades ranked by payback period: heat pump, insulation, windows, and more, with country-correct incentives applied (US IRA 30% federal tax credits, UK ECO4/Boiler Upgrade Scheme, EU/DE BEG grants, Canada Greener Homes, Australia state rebates). For homeowner and energy agents. Returns cost estimates, annual energy-bill savings, and climate-zone guidance by home type, locat