Introduction
At SolarInvestmentCalculator, we believe in transparency. This page details the authoritative data sources and key assumptions that drive our solar calculators, ensuring you understand the foundation of our estimates.
Data Sources
Solar Irradiance Data
Our solar irradiance values come from:
- National Renewable Energy Laboratory (NREL): PVWatts Calculator data
- NSRDB (National Solar Radiation Database): Historical solar radiation data
- Climate.One.Building.Solar (CBS): International solar radiation data
These sources provide average daily solar irradiance values in kWh/m²/day for locations across the United States.
Electricity Rate Data
Residential electricity rates by state are sourced from:
- U.S. Energy Information Administration (EIA): State-level residential electricity price data
- Utility Rate Database: Detailed rate structure information
Data is updated annually to reflect current rates. Note that actual rates may vary based on your specific utility company and rate plan.
Equipment Cost Data
Solar equipment costs are based on:
- SEIA (Solar Energy Industries Association): Quarterly solar market insight reports
- Bloomberg New Energy Finance (BNEF): Solar equipment price indices
- Lawrence Berkeley National Laboratory (LBNL): Tracking the Sun report series
Costs reflect national averages and include panels, inverters, racking, and installation labor.
Incentive Programs
Information about federal, state, and local incentives comes from:
- Database of State Incentives for Renewables & Efficiency (DSIRE): Comprehensive database of clean energy policies
- Energy.gov: Official federal incentive information
- State energy offices: Local incentive program details
Key Assumptions
System Performance
- Panel Efficiency: 18% for monocrystalline panels (typical for premium residential systems)
- Inverter Efficiency: 95% (representative of modern string inverters)
- System Losses: 14% (accounts for soiling, shading, temperature, mismatch, and other losses)
- Performance Ratio: 0.86 (ratio of actual to theoretical energy output)
Equipment Lifespan
- Solar Panels: 25-30 year useful life with linear degradation
- Inverters: 12-15 year lifespan (replacement assumed once during system lifetime)
- Mounting System: 25+ year lifespan
- Performance Guarantee: 80% output after 25 years for panels
Financial Assumptions
- Electricity Rate Inflation: 2.5% annually (historical average)
- System Degradation: 0.5% per year after initial stabilization
- Federal Tax Credit: 30% through 2032 (solar ITC schedule)
- Discount Rate: 3% for NPV calculations (conservative estimate)
- Maintenance Cost: $15-25 per kW annually after year 10
Location-Based Factors
Our location factors account for:
- Solar Resource: Average daily sun hours by geographic location
- Local Incentives: State and utility-specific rebate programs
- Interconnection Costs: Varies by utility and system size
- Building Codes: Regional variations in installation requirements
Methodology
Energy Production Calculation
We use a modified version of the industry-standard equation:
Energy (kWh) = STC Rating (kW) × Solar Irradiation (kWh/m²/day) × Performance Ratio × Days
The Performance Ratio incorporates all system losses and real-world operating conditions.
Financial Modeling
Our financial calculations consider:
- Time value of money using appropriate discount rates
- Escalating electricity costs over the system lifetime
- Gradual system degradation
- Impact of tax incentives on cash flows
- Salvage value of equipment at end of analysis period
Quality Assurance
Validation Process
Our calculators undergo rigorous validation:
- Comparison with industry-standard tools (PVWatts, SAM)
- Verification against real-world performance data
- Review by solar industry professionals
- Regular updates to reflect market changes
Accuracy Testing
We continuously test our calculators against:
- Third-party solar analysis tools
- Real installation performance data
- Updated cost and incentive information
- User feedback and reported discrepancies
Limitations
While we strive for accuracy, our calculators have limitations:
- Estimates are based on averages and may not reflect your specific situation
- Local conditions (shading, roof orientation) are not fully captured
- Future policy changes cannot be predicted
- Extreme weather events are not factored in
- Market fluctuations may affect equipment costs
Updates and Currency
We regularly update our data sources and assumptions:
- Electricity rates: Updated quarterly
- Equipment costs: Updated monthly
- Incentive programs: Updated as announced
- Solar irradiance: Updated annually
Last comprehensive update: December 2025
Conclusion
Our calculators are built on authoritative data sources and industry-standard assumptions. While they provide valuable estimates for planning purposes, we recommend validating results with professional solar installers who can assess your specific situation in detail.