Methodology

1. Annual consumption = monthly bill ÷ (rate in $/kWh) × 12.
2. System size = consumption × target offset (90%) ÷ local production per kW.
3. Annual production = system size × production per kW × roof-sun multiplier.
4. Gross cost = system watts × installed cost per watt.
5. Net cost = gross cost × (1 − federal credit) − state rebate. Federal credit defaults to 0 for 2026.
6. First-year savings = self-consumed kWh × rate + exported kWh × rate × export multiplier.
7. Simple payback = net cost ÷ first-year savings.
8. 25-year projection escalates savings with utility rates, reduces them by panel degradation, and subtracts annual maintenance plus a mid-life inverter replacement.

• Target bill offset: 90%.
• Roof-sun multipliers: poor 0.75, average 0.90, good 1.00, excellent 1.08.
• Export credit: full retail 1.0, partial/net billing 0.65, low/avoided cost 0.35, or custom.
• Self-consumption: 45% without a battery, 70% with one.
• Panel degradation: 0.5%/year. Electricity escalation: 3.5%/year (2%–5% sensitivity band).
• Installed cost default $3.00/W (range $2.40–$3.80). Inverter replacement $1,800 at year 12. Maintenance $150/year.
• Federal residential tax credit: 0% for post-2025 installations.

• ≤ 7 years: Likely strong
• 7–11 years: Potentially worthwhile
• 11–15 years: Mixed
• > 15 years (or no payback): Weak unless rates rise or costs fall
• Low data confidence adds "verify local utility terms."

Electricity prices come from the U.S. EIA, solar production from NREL PVWatts v8, the federal credit framing from the IRS, and incentive/net-metering context from DSIRE. See data sources for the live sync status of each. When live data is unavailable, we use clearly labeled representative fallbacks and mark confidence as low.

• Every output is an estimate, not a quote, appraisal, or tax determination.
• We model a state-representative location; your roof, shading, and utility differ.
• Net-metering and incentive rules change often and vary by utility within a state.
• We do not model time-of-use rate arbitrage, demand charges, or specific battery dispatch.