Complete Guide to Solar Panel ROI Calculation
A homeowner in Austin, Texas pays $200/month for electricity. At $0.165/kWh, that's 1,212 kWh/month. An 8 kW solar system on a south-facing roof at 30° tilt produces ~12,000 kWh/year. With Texas's full retail net metering, that's about $1,980 in annual savings. At $2.55/W installed ($20,400 gross, $14,280 after the 30% ITC), payback is ~7.2 years. Over 25 years, accounting for 0.5% annual degradation and 2.5% utility rate inflation, total savings exceed $52,000. But in California under NEM 3.0, the same system saves only ~$1,200/year because exports earn 5–8¢/kWh. The difference? Net metering policy.
Why Your Address Determines Everything
Solar ROI has three geographic drivers: (1) solar irradiance — Phoenix gets 6.2 kWh/m²/day, Seattle 3.9. (2) Electricity rates — Hawaii pays $0.42/kWh, North Dakota $0.12. (3) Net metering — California NEM 3.0 slashed export credits by ~75%, pushing payback from 6 to 10+ years. PVWatts uses your lat/lon to pull the nearest NSRDB weather station and run an hour-by-hour simulation against 20 years of data.
| City | kWh/m²/day | Rate | NEM | Annual kWh | Payback | 25-yr Savings |
|---|---|---|---|---|---|---|
| Phoenix, AZ | 6.2 | $0.156 | Reduced | 13,800 | 8.2 yr | $38,000 |
| Austin, TX | 5.4 | $0.164 | Full retail | 12,000 | 7.6 yr | $52,000 |
| Seattle, WA | 3.9 | $0.144 | Full retail | 8,400 | 9.8 yr | $28,000 |
| Honolulu, HI | 5.5 | $0.422 | Reduced | 11,200 | 5.1 yr | $78,000 |
| San Francisco, CA | 5.6 | $0.334 | NEM 3.0 | 10,800 | 10.5 yr | $22,000 |
How NREL PVWatts v8 Works
PVWatts is the industry-standard solar performance model from the U.S. Department of Energy. Version 8 (Nov 2022) uses 2020 TMY weather data and includes bifacial modules, monthly soiling and an updated thermal model. Inputs: location, system size, tilt, azimuth, module type. Outputs: monthly and annual AC production, capacity factor and plane-of-array irradiance. The model accounts for temperature (panels lose ~0.4%/°C above 25°C), soiling, wiring and inverter losses.
Annual Savings = (Production × Self-Consumption × Retail Rate) + (Production × Export % × Export Rate)
Interactive Scenarios
What if your roof faces east instead of south?
What if you add a 10 kWh battery in California?
What if rates rise 4% instead of 2.5%?
Understanding Solar Panel Degradation
- P-type PERC (standard): 0.5–0.7%/yr, 84–86% at year 25
- N-type TOPCon (premium): 0.35–0.5%/yr, 88–92% at year 25
- First-year LID: 1.5–3% (P-type), 0.5–1% (N-type)
- Hot desert climates degrade 0.8–1.2%/yr; temperate 0.4–0.6%/yr
Methodology & Data Sources
- NREL PVWatts v8 model — irradiance, tilt and azimuth derates calibrated to PVWatts outputs.
- EIA residential rates — March 2026 state averages (¢/kWh).
- Net metering tiers — DSIRE database and state utility commission filings.
- Solar cost benchmark — NREL Q1 2026 residential, national $2.58/W.
- Degradation — NREL 2024 meta-analysis of 54,000 systems.