Solar Access and TSRF tell you how much usable sun a site actually delivers to your modules and how close your array is to the location’s potential. Mastering these metrics lets you forecast energy yield, set accurate expectations, and prioritize designs that win. Below you will find clear formulas, quick examples, and practical guidance so you can perform a robust solar access and TSRF calculation on every project.
From irradiance to energy: essential terms
Irradiance is instantaneous solar power on a surface (W/m²). Insolation is the total energy over a period (Wh/m² or kWh/m²). Plane-of-array (POA) irradiance is the irradiance on the tilted module surface. Shading reduces POA at specific times and azimuths, which is why time-resolved POA is the foundation for solar access and TSRF.
Calculating Solar Access (%)
Solar Access expresses how much sun a surface receives compared to the same surface with no shade, over a defined period. It is computed by summing shaded POA irradiance and dividing by unshaded POA, often hour by hour across the year. For a given module or roof face, Solar Access = Shaded POA / Unshaded POA. If a surface receives 950 kWh/m² while the same unshaded surface would receive 1,000 kWh/m², Solar Access is 95%.
Tilt and Orientation Factor (TOF)
TOF captures how your array’s tilt and azimuth impact available POA compared to the site’s ideal orientation. TOF = Actual unshaded POA of your array / Optimal unshaded POA at that location. The optimal orientation is location dependent, typically near south-facing in the Northern Hemisphere with a tilt around local latitude. A TOF of 0.92 means your orientation and tilt deliver 92% of the site’s ideal POA.
Total Solar Resource Fraction (TSRF)
TSRF combines orientation effects and shading into one metric used for fast site qualification and incentive or interconnection thresholds. You can compute TSRF in two equivalent ways: TSRF = Shaded POA / Optimal unshaded POA, or TSRF = TOF × Solar Access. For example, if TOF is 0.92 and Solar Access is 95%, TSRF is 0.874 or 87.4%. During applications, some authorities and utilities request shade/TSRF documentation—maintain clear, consistent records of assumptions and methods.
Why it matters: TSRF closely tracks how much solar resource your modules actually see, so it is a strong predictor of production compared to a perfect, shade-free, ideally oriented array at the same location. As a rule of thumb, many residential rooftops targeted for strong performance land above roughly 75-80% TSRF, while heavy tree shading or poor azimuth can push TSRF lower. Improving TSRF often means moving array placements, adjusting tilt, or mitigating shade where feasible.
Measuring these metrics accurately
Accurate solar access and TSRF hinge on reliable geometry and time-resolved shading. Start with a thorough solar site assessment. In practice, teams use a combination of high-quality 3D roof models, horizon obstructions, and a shade engine that samples the sun path across the year. Handheld shade tools can provide snapshots but may vary by operator and point count, while software modeling standardizes sampling and enables what-if scenarios across roof faces and module layouts. If you’re clarifying responsibilities and deliverables across teams, review site survey vs. site assessment.
With Enervio, you can streamline the front end of this workflow – remote property characteristic collection, 3D visualization with sun and shade simulations, AI-guided education, and one-click quotes. Use consistent geometry and shade insights to inform your preferred solar access and TSRF calculation method, then carry those results into bankable production estimates. During homeowner conversations, use these questions to ask during a solar consultation to collect details that affect shading and orientation. For onsite verification of shading and production assumptions, consult our solar site inspections guide.
Quick reference: formulas and units
| Metric | Formula | Units |
|---|---|---|
| Solar Access | Shaded POA / Unshaded POA | % or fraction |
| TOF | Actual unshaded POA / Optimal unshaded POA | % or fraction |
| TSRF | Shaded POA / Optimal unshaded POA = TOF × Solar Access | % or fraction |
| Irradiance vs. Insolation | Power vs. energy over time | W/m² vs. kWh/m² |
FAQs
How do you calculate solar access?
Simulate time-resolved POA on the target surface with and without shading, then divide shaded by unshaded totals for the same period. Express the result as a percentage for the module, roof face, or entire array.
How to calculate TSRF?
Use TSRF = Shaded POA / Optimal unshaded POA, or multiply TOF by Solar Access. Derive POA from hourly or sub-hourly simulation so orientation and shade effects are captured across the full year.
What is the 33% rule in solar panels?
It commonly refers to a design heuristic where the DC rating is up to about 33% higher than the inverter AC rating to reduce cost while keeping clipping acceptable. Always validate with climate, inverter specs, and codes.
What is the solar 120% rule?
In many jurisdictions following NEC, the 120% rule limits busbar current. The sum of the main breaker plus PV backfeed breaker cannot exceed 120% of the busbar rating. Check local code and utility requirements.
If you want a faster path from site screening to a confident proposal, Enervio helps you capture accurate property data, visualize shade, educate prospects, and generate quotes – so your solar access and tsrf calculation feeds directly into compelling, trusted offers. To present TSRF and solar access clearly in your offers, use our solar proposal templates and best practices.
