Methodology & Data Sourcing
At GoSolarIndex, every calculation, payback timeline, and hardware score is open, reproducible, and grounded in verifiable federal energy datasets and standard electrical engineering formulas.
1. Authoritative Data Sources
Rather than relying on proprietary contractor estimates or unverified marketing figures, our data pipeline pulls directly from official government and laboratory repositories:
| Dataset / Agency | Metric Sourced | Update Cadence |
|---|---|---|
| EIA Form 861MUS Energy Information Administration | State-level residential electricity rates (cents per kilowatt-hour) and monthly consumption. | Monthly |
| NREL PVWatts v8National Renewable Energy Laboratory | Solar irradiance, peak sun hours per day, and annual generation per DC kilowatt capacity. | Annual |
| LBNL Tracking the SunLawrence Berkeley National Laboratory | Median installed cost per watt ($/W) for residential grid-tied photovoltaic arrays. | Annual |
| DSIRE USANC Clean Energy Technology Center | State net metering statutes, property tax exemptions, SREC markets, and solar access rights. | Continuous |
| Direct Manufacturer SpecsUL 9540, UL 1741 & CEC Datasheets | Usable kilowatt-hour storage, continuous kilowatt rating, surge wattage, round-trip efficiency, and warranty cycles. | Per release |
2. Core Calculation Formulas
A. Annual Solar Generation & Sizing
Annual solar generation is modeled using location-specific peak sun hours with an engineering derate factor:
Derate Factor (0.86): A standard 14% total system loss accounts for DC-to-AC inverter conversion efficiency (~97%), wiring and connection losses (~2%), module soiling/dust (~2%), and thermal coefficient derating during elevated ambient summer temperatures (~5-7%).
B. Panel Count & Roof Area Footprint
Rooftop requirements are calculated from modern residential module specs and building safety standards:
Roof Area (sq ft) = Panels × 20 sq ft per module
Module Reference: Standard 66-cell / 120-half-cell residential modules measure approximately 68.5 inches by 40.5 inches (19.3 sq ft). Factoring in racking clamps, thermal expansion gaps, and International Fire Code (IFC 1205) 36-inch ridge and valley setbacks, each module requires 20 square feet of unobstructed roof space.
C. Battery Storage & Surge Capacity
Required battery nameplate capacity and continuous inverter power are sized to maintain essential loads through blackouts:
Nameplate kWh = Usable kWh / Depth of Discharge (DoD)
Compressor Surge: Inductive loads (HVAC, well pumps, refrigerators) draw Locked Rotor Amperage (LRA) lasting 200–500 milliseconds that is 3x to 6x higher than steady-state running wattage. Our calculators verify that the selected battery system's peak inverter surge rating exceeds the starting requirements of all active inductive circuits.
D. Payback Period & Net Metering Compensation
Payback models calculate real bill offsets based on local utility tariff structure:
Annual Bill Savings = (Self-Consumed kWh × Retail Rate) + (Exported kWh × Export Credit Rate)
Tariff Modeling: In retail 1:1 net metering states, exported power is credited at full retail rate. In net billing states (such as California under NEM 3.0), exported power is credited at the utility avoided-cost rate (typically 5 to 8 cents per kWh). Our calculator illustrates why pairing batteries with solar in net billing markets dramatically improves payback by shifting stored solar energy into high-rate evening peak hours.
E. Conductor Cable Sizing & Voltage Drop
Conductor calculations adhere strictly to National Electrical Code (NEC) guidelines:
NEC 210.19(A) & 215.2(A): Total combined voltage drop across branch circuits and feeders should not exceed 3% for optimal efficiency and to prevent premature inverter tripping or appliance damage.
3. Federal Tax Law Compliance
Unlike outdated solar websites that still present universal 30% homeowner tax credits, GoSolarIndex models the legal realities of current federal tax legislation:
- Direct Homeowner Ownership (IRC §25D): The residential clean energy credit under Internal Revenue Code Section 25D expired at the close of 2025 for homeowner-purchased systems. Our calculator defaults to 0% federal credit for direct homeowner cash and loan purchases, ensuring realistic financial modeling.
- Third-Party Leases & PPAs (IRC §48E): Commercial clean electricity investment credits under Section 48E remain available for commercial system owners (such as Sunrun, Sunnova, or installer-owned systems). When comparing leases and PPAs, we model how installers monetize this commercial credit and pass savings through lower monthly rates.
- State & Local Rebates: We independently track state solar rebates, property tax exemptions, sales tax holidays, and Solar Renewable Energy Certificate (SREC) markets across all 50 states and D.C.
4. Hardware Scoring & Review Criteria
Our battery and power station comparisons evaluate products using objective engineering parameters:
Energy Density & Chemistry
Evaluation of lithium iron phosphate (LiFePO4) vs nickel manganese cobalt (NMC) chemistries, thermal stability, usable capacity ratio, and weight-to-watt-hour performance.
Inverter & Surge Rating
Assessment of continuous kilowatt output, pure sine wave total harmonic distortion (THD < 3%), and surge overhead required to start high-inductance electric motors.
Lifecycle & Warranty
Guaranteed cycle life to 70% or 80% retained capacity, warranty period in years, throughput guarantees (MWh), and manufacturer reliability record.
5. Corrections & Data Review
Utility tariffs and state incentive rules change regularly. If you identify a newly approved utility rate case, an altered net metering tariff, or a local incentive revision, please submit the docket or documentation via ourContact page. Our engineering team reviews and updates affected state models within 48 business hours.