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How this is calculated (no black box): Gas heat energy = annual gas spend ÷ gas price, converted to the heat delivered at your AFUE. The same delivered heat via a heat pump draws (heat ÷ COP) kWh, priced at your $/kWh. Annual change = gas spend − heat-pump electricity cost. Net premium = tonnage × (incremental $/ton − rebate). Payback = net premium ÷ annual savings.

Honest caveats: This is a screening estimate, not an energy model. Real payback swings with climate, electricity vs. gas price ratio, and grid — DOE modeling of the same unit showed roughly +27% bill savings in San Francisco versus about −25% in Minnesota. It excludes electrical-service upgrade costs (all-electric backup can raise amp draw). Section 179D is terminated for construction beginning after June 30, 2026; verify MACRS depreciation and current utility rebates with a qualified professional.