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IS 11907:1986 is the Indian Standard (BIS) for recommendations for calculation of solar radiation on buildings. This standard provides a methodology for calculating the solar radiation incident on building surfaces. It includes fundamental formulas for solar angles, components of solar radiation (direct, diffuse, reflected), and provides tabulated climatic data for 14 major Indian cities to aid in design.
Recommendations for calculation of solar radiation on buildings
Overview
Status
Current
Usage level
Specialized
Domain
Architectural — Functional Requirements in Buildings
BIM-relevant code. See the BIM Hub for ISO 19650, IFC, and LOD/LOIN frameworks used alongside it.
Practical Notes
! The climatic data in Annex A is from before 1986. For accurate energy simulations, it is highly recommended to use recent Typical Meteorological Year (TMY) or weather data files for the specific location.
! This code forms the basis for calculating Solar Heat Gain Coefficient (SHGC) and thermal performance of building envelopes as required by the National Building Code (NBC) and Energy Conservation Building Code (ECBC).
! Be careful with the sign conventions and definitions for angles (e.g., solar azimuth, wall azimuth) as they are critical for correct calculations.
Provides authoritative methods and data for calculating solar radiation and angles on building surfaces for load calculations.
ISO 52010-1:2017ISO (International Organization for Standardization), International
HighCurrent
Energy performance of buildings — External climatic conditions — Part 1: Conversion of climatic data for energy calculations
Specifies the calculation of solar irradiance on surfaces of any orientation and tilt from meteorological data for energy analysis.
CIBSE Guide A: 2015CIBSE (Chartered Institution of Building Services Engineers), UK
HighCurrent
Guide A: Environmental Design (Chapter 2: External design data & Chapter 5: Thermal properties of building structures)
Includes detailed procedures and UK-specific data for calculating solar radiation for building thermal and daylighting analysis.
Key Differences
≠The Indian standard (1986) provides solar radiation data specific to 15 Indian cities for 'clear-sky' design days. Modern international standards use comprehensive hourly weather data files (e.g., TMY, EPW) for a wide range of global locations, covering all sky conditions.
≠IS 11907:1986 implicitly uses an isotropic model for calculating diffuse radiation on a tilted surface. Modern standards like ASHRAE and ISO 52010 recommend more accurate anisotropic models (e.g., the Perez model) that account for circumsolar and horizon brightening.
≠The Indian standard is a concise, standalone document primarily for manual calculations. International equivalents like the ASHRAE Handbook and ISO 52010 are part of a larger, integrated framework for building energy performance and are designed for use with computer simulations.
≠Due to its age, IS 11907:1986 is based on models and atmospheric data from before the 1980s. International standards are regularly updated to incorporate newer research, more accurate atmospheric data, and improved calculation models.
Key Similarities
≈All standards are based on the same fundamental principles of solar geometry (spherical trigonometry) to calculate solar angles like altitude, azimuth, and the angle of incidence for any location and time.
≈The calculation methodology in all standards is component-based, breaking down total solar radiation on a surface into three parts: direct beam radiation, sky-diffuse radiation, and ground-reflected diffuse radiation.
≈All standards specify that solar radiation calculations must be performed using Local Solar Time, and they provide formulas to convert from local standard time by correcting for the Equation of Time and longitude.
≈The fundamental objective is identical: to provide a standardized procedure for quantifying solar radiation on building surfaces, which is essential for calculating thermal loads, sizing HVAC systems, and designing shading devices.
Parameter Comparison
Parameter
IS Value
International
Source
Solar Constant
1353 W/m² (This value is not explicitly stated but was the WRC standard at the time and is the basis for the included data).
1361 W/m² (Average over a year).
ASHRAE Handbook—Fundamentals 2021
Model for Diffuse Radiation on Tilted Surface
Isotropic sky model, where diffuse radiation is uniform across the sky dome.
Anisotropic sky model (Perez model), which accounts for non-uniform sky luminance.
ISO 52010-1:2017
Clear Sky Irradiance Model
Based on tabulated atmospheric extinction coefficients for Indian cities.
ASHRAE Clear-Sky Model, using empirically derived coefficients A, B, and C that vary monthly.
ASHRAE Handbook—Fundamentals 2021
Standard Ground Reflectance (Albedo)
0.2 (for 'ground and surroundings').
0.2 (common default for ground/vegetation), with a wider table of values for other surfaces like snow, concrete, and water.
ASHRAE Handbook—Fundamentals 2021
Equation of Time (EoT) calculation
Provides a simplified formula: E = 9.87 sin(2B) - 7.53 cos(B) - 1.5 sin(B), where B = 360(n-81)/364.
Provides a more complex, higher-fidelity Fourier series approximation for EoT, or recommends using tabulated daily values.
ISO 52010-1:2017
⚠ Verify details from original standards before use
Key Values6
Quick Reference Values
Solar Constant1353 W/m²
Albedo of fresh snow0.80 to 0.85
Albedo of light colored concrete0.30 to 0.35
Albedo of green grass0.20 to 0.25
Atmospheric turbidity coefficient (B) for Industrial Atmosphere0.10
Atmospheric turbidity coefficient (B) for Clean Atmosphere0.05
Key Formulas
sin α = sin L sin δ + cos L cos δ cos h — Solar altitude angle (α)
sin γ = cos δ sin h / cos α — Solar azimuth angle (γ)
IT = IDN cos θ + Id + IR — Total solar radiation on a surface
Tables & Referenced Sections
Key Tables
Table 1 - Monthly Average Daily Global Solar Radiation on Horizontal Surface
Table 2 - Monthly Average Daily Diffuse Solar Radiation on Horizontal Surface
Table A-1 to A-14 - Solar Radiation and Related Data for Various Indian Cities
Key Clauses
Clause 4 - Solar Radiation
Clause 5 - Solar Angles
Clause 6 - Calculation of Solar Radiation on Surfaces
Annex A - Solar Radiation and Related Data for Design
Frequently Asked Questions4
What is the value of the solar constant used in this standard?+
The recommended value for the solar constant is 1353 W/m² (Clause 4.2).
Where can I find solar radiation data for Chennai?+
Annex A, Table A-8 provides detailed monthly solar data for Chennai.
What is the main formula to calculate solar altitude?+
sin α = sin L sin δ + cos L cos δ cos h, where α is altitude, L is latitude, δ is declination, and h is the hour angle (Clause 5.2.1).
Does this code provide reflectivity values for common ground surfaces?+
Yes, Clause 4.5.1 provides typical albedo (reflectivity) values for surfaces like concrete, grass, and water.