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How We Calculate Solar Cost Numbers
Every cost figure on SolarProGuide — from state cost tables to payback calculators — is built from four data sources. Here's exactly what we use and how we combine them.
Source 1: SEIA U.S. Solar Market Insight (Installer Pricing)
The Solar Energy Industries Association publishes quarterly reports showing installed residential solar pricing per watt by state and system size. We use their most recent quarterly snapshot as our baseline gross-cost figure, updated within 60 days of each SEIA release.
Source 2: NREL PVWatts (Production Estimates)
NREL PVWatts is the National Renewable Energy Laboratory's peer-reviewed solar production model. We use it to calculate expected annual kWh production per ZIP code, factoring in peak sun hours, tilt, azimuth, shading, and panel degradation. All state/city pages use PVWatts-derived production figures.
Source 3: EIA Electric Power Monthly (Rates + Savings)
U.S. Energy Information Administration publishes state-by-state average residential electricity rates monthly. We multiply PVWatts production × EIA rate to get bill savings. We assume a 3%/year rate escalation based on 10-year EIA historical trends.
Source 4: IRS Form 5695 + DSIRE (Incentives)
Federal tax credit rules come directly from IRS Form 5695 Instructions — currently 30% through 2032. State and utility incentives come from the DSIRE database, maintained by the NC Clean Energy Technology Center.
Reality Check: Real Installer Proposals
Modeled numbers are only useful if they match real quotes. Every quarter our editorial team reviews 20-40 anonymized installer proposals from the SolarProGuide network. When modeled figures drift more than ±10% from real quotes, we investigate and update.
Assumptions in Every Cost Table
- System type: grid-tied, no battery unless noted
- Panel efficiency: 20-22% (Tier-1 monocrystalline)
- System degradation: 0.5%/year
- Roof type: standard asphalt shingle, unshaded, south or west-facing
- Federal ITC: 30% (through 2032)
- Utility rate escalation: 3%/year
- Financing: cash purchase (unless "financed" is noted)
What We Don't Model (Yet)
- Real-time SREC or utility rebate market prices
- Time-of-use rate structures beyond simple net metering
- Battery cycling arbitrage under NEM 3.0
- Home value appreciation from solar (see LBNL "Selling Into the Sun")
Errors & Updates
If our numbers don't match real quotes you're seeing, please email [email protected] — we investigate every mismatch and update our modeling accordingly. Our goal is that a homeowner reading this site gets numbers within ±10% of what they'll actually see in a real installer proposal.