Solar Reflectance Index Calculator

Calculator Inputs

Material name

Solar reflectance

Thermal emittance

Solar irradiance, W/m²

Ambient temperature

Sky temperature

Temperature unit

Wind condition

Custom convection, W/m²K

Black reference reflectance

White reference reflectance

Reference emittance

Surface area

Area unit

Decimal precision

Notes

Example Data Table

Material	Reflectance	Emittance	Wind setting	Estimated SRI	Estimated surface temperature
White roof coating	0.72	0.90	Medium	88.69	49.03 °C
Aged roof membrane	0.55	0.85	Medium	63.64	58.50 °C
Gray paver	0.30	0.90	Medium	31.87	70.52 °C
Dark roof sheet	0.08	0.90	Medium	3.75	81.16 °C

Formula Used

Absorptance: α = 1 − ρ

Surface heat balance: αI = εσ(Ts⁴ − Tsky⁴) + hc(Ts − Ta)

Solar Reflectance Index: SRI = 100 × (Tblack − Tsample) / (Tblack − Twhite)

Here, ρ is solar reflectance, α is absorptance, ε is thermal emittance, σ is the Stefan-Boltzmann constant, I is solar irradiance, hc is convection coefficient, and T values are absolute temperatures.

How to Use This Calculator

Enter the material name for your roof, panel base, pavement, or coating.
Add solar reflectance and thermal emittance as decimal values.
Enter solar irradiance, ambient temperature, and sky temperature.
Select a wind condition or enter a custom convection coefficient.
Keep default black and white references unless your standard requires changes.
Add the surface area to estimate absorbed solar power.
Press calculate to show results above the form.
Use CSV or PDF buttons to download the same input report.

Solar Reflectance Index Overview

Solar Reflectance Index, or SRI, rates how cool a surface stays under sun. It combines solar reflectance and thermal emittance into one practical value. A high score means the material reflects more sunlight and releases absorbed heat well. A low score means the surface can become hotter and may add heat to nearby rooms, panels, conduits, and roof equipment.

Why SRI Matters

Roofs, paving, and equipment pads often sit near cables, ducts, inverters, and service spaces. Heat from dark surfaces can raise surrounding air temperature. That can reduce comfort and may increase cooling demand. SRI helps designers compare finish choices before buying materials. It is also useful when checking cool roof targets, green building documents, and product submittals.

About the Inputs

Solar reflectance is entered from 0 to 1. A value of 0.70 means seventy percent of sunlight is reflected. Thermal emittance is also entered from 0 to 1. It shows how well the surface radiates heat. Ambient temperature, sky temperature, solar irradiance, and wind level shape the final surface temperature estimate.

How the Calculator Works

This calculator solves the steady heat balance for the selected material. It also solves the same balance for black and white reference surfaces. The final SRI compares your surface temperature between those two references. A value near 100 behaves close to the white reference. A value near 0 behaves close to the black reference. Values below 0 or above 100 can occur for unusual products or custom conditions.

Practical Use

Use measured reflectance and emittance when available. Product sheets may list aged values, initial values, or both. Aged values usually better represent long term roof performance. For design comparisons, keep ambient temperature, sky temperature, solar irradiance, and wind condition the same across all options.

Result Interpretation

SRI is not a complete energy model. It does not include insulation, roof slope, shading, thermal mass, or hourly weather. Still, it gives a clear material-level comparison. Use it with cost, durability, glare, safety, and maintenance checks. The exported CSV and PDF help document choices for clients, engineers, and project files. They also make repeated reviews easier when several materials are tested under identical project assumptions during early design stages and procurement planning.

FAQs

What is Solar Reflectance Index?

Solar Reflectance Index is a rating that compares how hot a surface becomes under sunlight. It combines solar reflectance and thermal emittance into one value.

Is a higher SRI better?

Yes, a higher SRI usually means the surface stays cooler. It reflects more solar energy and releases absorbed heat more effectively.

Can SRI be greater than 100?

Yes. Some very reflective or highly emissive materials can exceed the white reference. Custom conditions may also push values above 100.

Can SRI be negative?

Yes. A negative value means the sample can become hotter than the black reference under the entered conditions.

What reflectance value should I enter?

Use the measured solar reflectance from a product sheet or test report. Aged values are often better for long-term roof checks.

What emittance value should I enter?

Use the material thermal emittance from reliable product data. Most roofing products list this value between 0 and 1.

Why does wind condition affect SRI?

Wind changes convective heat loss. Higher wind removes heat faster, which changes calculated surface temperature and final SRI.

Is this a full building energy model?

No. This calculator compares material surface behavior. It does not model insulation, occupancy, shading, HVAC schedules, or hourly weather.