Solar Panel kW Sizing, Lithium Battery & Payback ROI Calculator

Solar + Battery Payback Calculator

Solar panels are a 25-year power plant on your roof, and the only question that matters is the payback period: total installed cost minus incentives, divided by annual electricity savings. A typical 8 kW US system at ~$2.50–$3.50/watt costs $20,000–$28,000 before the 30% federal tax credit, then saves $1,500–$2,500 a year — putting breakeven around 6–9 years, with 15+ years of nearly free electricity after. The math is genuinely good in sunny states with high utility rates; it is marginal where power is cheap and clouds are frequent.

This calculator builds your specific case: system size, local sun hours, your actual per-kWh rate (and its annual increase — utilities raise rates ~3–4% yearly, which quietly shortens payback), roof orientation, plus battery storage if you want backup. Batteries change the economics: they add $8,000–$12,000 but mostly buy resilience and time-of-use arbitrage rather than raw payback, so the calculator shows solar-only versus solar+battery separately.

The two inputs people get wrong: shading and rate plans. Even partial afternoon shade can cut production 20–30%, and switching to a time-of-use plan can either supercharge savings (export at peak rates) or hurt them. Get a proper shade analysis and model your utility's actual solar rate structure — net metering 3.0 regimes pay far less for exported power than the old 1:1 plans, which shifts optimal system sizing toward self-consumption.