Indiana Solar Irradiance and Sun Hours Data

Indiana receives a moderate solar resource that varies significantly by season, latitude, and local cloud cover patterns. This page covers the state's measured irradiance values, peak sun hour averages by region, how those figures feed into system sizing calculations, and what distinguishes Indiana's solar climate from neighboring states. Understanding this data is foundational to evaluating whether a proposed installation will generate enough energy to meet a project's financial and performance targets.

Definition and scope

Solar irradiance is the power of solar radiation received per unit area, measured in watts per square meter (W/m²). Peak sun hours (PSH) represent the equivalent number of hours per day during which irradiance averages 1,000 W/m² — the standard test condition (STC) used to rate photovoltaic (PV) panels. A location recording 4.5 PSH does not mean the sun shines for exactly 4.5 hours; it means the total daily solar energy equals what 4.5 hours of full-intensity irradiance would deliver.

The primary dataset used for U.S. locations is the National Solar Radiation Database (NSRDB), maintained by the National Renewable Energy Laboratory (NREL). NREL's PVWatts Calculator draws on NSRDB records to provide location-specific estimates for any Indiana address. The Solar Resource Data is also referenced in permitting documentation reviewed by the Indiana Utility Regulatory Commission (IURC) and by utilities administering interconnection agreements under IURC jurisdiction.

Scope and coverage limitations: This page applies exclusively to Indiana's geographic boundaries. It does not address solar resource data for adjacent states (Ohio, Michigan, Illinois, Kentucky) or federal land classifications. It does not constitute an engineering report, and it does not supersede site-specific measurements from calibrated pyranometers or on-site data loggers. System designers subject to National Electrical Code (NEC) Article 690 requirements under NFPA 70 (2023 edition, effective 2023-01-01) and local Authority Having Jurisdiction (AHJ) review must use project-specific irradiance data, not statewide averages alone.

How it works

Indiana spans roughly 2.7 degrees of latitude (from approximately 37.8°N at the Kentucky border to 41.8°N near Lake Michigan), producing a measurable north-to-south gradient in annual solar resource.

Annual average peak sun hours by region (NREL NSRDB-derived):

  1. Northern Indiana (South Bend, Fort Wayne corridor): approximately 4.0–4.2 PSH/day annual average, with lake-effect cloud cover from Lake Michigan suppressing totals particularly in November through February.
  2. Central Indiana (Indianapolis metro): approximately 4.4–4.6 PSH/day annual average; Indianapolis sits near the state mean and is the most commonly cited reference point in utility and state energy planning documents.
  3. Southern Indiana (Evansville, Jeffersonville): approximately 4.6–5.0 PSH/day annual average, benefiting from lower latitude and reduced Great Lakes cloud influence.

Global Horizontal Irradiance (GHI) is the most commonly used irradiance metric for flat-panel installations. Direct Normal Irradiance (DNI) applies to concentrating solar systems, which are rare in Indiana's distributed generation market. Diffuse Horizontal Irradiance (DHI) captures scattered sky radiation and becomes proportionally more important in overcast climates — relevant to northern Indiana where diffuse fractions are higher.

Seasonal variation is pronounced. Indianapolis averages roughly 5.8–6.2 PSH/day in June and July, dropping to approximately 1.8–2.2 PSH/day in December and January (NREL PVWatts v8). This 3:1 seasonal swing directly affects annual production estimates and must be reflected in the Indiana solar system sizing methodology used during project design.

Panel orientation and tilt interact with irradiance values. In Indianapolis (latitude ~39.8°N), a south-facing array tilted at 30–35 degrees captures close to the maximum annual GHI. Deviations of 45 degrees from true south or tilt angles below 10 degrees can reduce annual yield by 5–15%, based on NREL modeling outputs.

Common scenarios

Residential rooftop systems: Most Indiana residential installations are sized using a 12-month consumption baseline from the utility and a location-adjusted PSH figure. A home consuming 10,000 kWh/year in Indianapolis, with system losses of approximately 14% (wiring, inverter, soiling — NEC 690.8 under NFPA 70 2023 edition requires worst-case calculations), would need roughly 6.5–7.5 kW of DC nameplate capacity. The PSH input directly controls this calculation. Installers often cross-reference NREL PVWatts outputs with Indiana solar panel performance in Midwest climate considerations to verify temperature derating.

Agricultural and ground-mount systems: Rural Indiana installations — covered in more detail at ground-mount solar systems in Indiana and Indiana agricultural solar installations — typically use NREL TMY3 (Typical Meteorological Year 3) or TMY data for financial modeling. Lenders and state agricultural finance agencies may require NREL-sourced irradiance documentation before approving project financing.

Community solar and utility-scale projects: Projects connected at the distribution or transmission level, reviewed under IURC dockets and utility interconnection tariffs, incorporate irradiance data into capacity factor calculations. A utility-scale PV plant in southern Indiana typically achieves a capacity factor of 15–18%, compared to 13–15% in the northern tier — a distinction with material impact on power purchase agreement pricing. The Indiana utility interconnection requirements page covers how generation estimates feed into interconnection study submissions.

Battery storage integration: Irradiance data shapes storage sizing. Systems designed to maximize self-consumption during low-irradiance winter months require larger battery banks than summer-optimized designs. Indiana solar battery storage integration addresses how seasonal production curves interact with storage scheduling logic.

Decision boundaries

Irradiance data drives three discrete decision thresholds in Indiana solar project development:

  1. Feasibility threshold: Projects where NREL-modeled annual production falls below the break-even consumption level for the proposed array size are typically reconsidered for scope, orientation adjustment, or tree-removal assessment before advancing to permit application.
  2. Permitting documentation: Local AHJs — operating under Indiana Building Code references to NEC Article 690 as adopted under NFPA 70 2023 edition (effective 2023-01-01) — frequently require a completed PVWatts report or equivalent irradiance-based production estimate as part of the electrical permit package. The permitting and inspection concepts for Indiana solar energy systems framework details which documents AHJs typically accept.
  3. Incentive qualification: Federal Investment Tax Credit (ITC) calculations under Internal Revenue Code §48(e) and §25D do not require irradiance documentation, but state-level programs and utility rebates administered under IURC authority may require production modeling grounded in NSRDB data.

Comparing Indiana to Illinois: Chicago averages approximately 3.9–4.1 PSH/day at its northern latitude, while southern Illinois approaches Indiana's southern-tier figures. Indiana's central corridor is modestly superior to Chicago but meaningfully below the 5.5–6.0 PSH/day averages found in the American Southwest. This positions Indiana as a viable but not exceptional solar market — one where panel efficiency selection, covered in Indiana solar panel brands and equipment options, carries more weight than in high-irradiance states.

For a broader orientation to how irradiance fits within the full solar development workflow, the conceptual overview of how Indiana solar energy systems work provides context, and the Indiana Solar Authority home indexes the full body of state-specific solar reference material. The regulatory context for Indiana solar energy systems page addresses how IURC rules and utility tariffs interact with production data requirements.

References

📜 3 regulatory citations referenced  ·  ✅ Citations verified Feb 25, 2026  ·  View update log

Explore This Site

Services & Options Types of Indiana Solar Energy Systems Regulations & Safety Regulatory Context for Indiana Solar Energy Systems Tools & Calculators Solar Battery Calculator FAQ Indiana Solar Energy Systems: Frequently Asked Questions