Solar Factor: What It Is and How to Calculate It

Window glass is not just an element that separates the inside from the outside, but it is a fundamental component that can influence thermal insulation, natural lighting, and consequently, living comfort. Choosing the right glass can help reduce energy costs, improve thermal efficiency, and optimize the intake of natural light.

To understand the performance of glass, there are parameters to consider such as the solar factor and light transmission (TL). The solar factor helps us understand how much solar energy passes through the glass surface. Light transmission, on the other hand, represents the amount of light that the glass of a fixture allows to filter inside.

The solar factor is a determining parameter in the design of a building, especially when the environments have large transparent surfaces, as it is able to quantify the heat that enters the internal environments. Therefore, it is an important value in the choice of frames and windows as it influences comfort and energy savings within buildings.

Solar factor and glass

To understand what the solar factor indicates, one must first know what happens when solar radiation hits a glass.

Glass, by its nature, is considered transparent to light, but it is only partially transparent to solar radiation. When a ray of sunlight hits a glass surface, the following phenomena occur:

• reflection: part of the total incident solar radiation is reflected by the glass;
• transmission: part of the total incident solar radiation passes through the glass;
• absorption: part of the solar radiation is absorbed by the glass;
• emission: part of the incident solar radiation absorbed by the glass is gradually released into the surrounding environment.

The measured values for each of these phenomena, which are interrelated, determine the so-called solar gain or solar transmission, indicated by the parameter g.

This value, which ranges from 0 to 1, indicates the ratio between the amount of solar radiation transmitted inside the building and the total radiation incident on the outer surface of the glass.

A value tending to zero indicates a lower capacity of the glass for solar transmission, while a value close to one indicates a high transmission capacity.

What is the solar factor

The solar factor, also known as the solar energy gain coefficient or total energy transmission, is a value that measures the amount of solar energy that can pass through a glass element. It is therefore able to quantify the heat that enters the internal environments and is of great importance in determining the overall energy balance.

In the winter months, in fact, free solar gains positively contribute to reducing the energy demand for heating. Conversely, in the summer season, if not properly considered during the design phase or during the choice of glazing products for construction, they can create overheating problems, particularly for Mediterranean countries.

The solar factor g of a glass surface, therefore, represents the percentage of thermal energy, τ_e, that directly passes through the pane compared to the total incident energy. It is an index expressed as a percentage % and is useful for evaluating the energy performance of a glass element.

The value of this parameter can vary greatly, ranging from a minimum of 0.1 to a maximum of 0.9 (10% – 90%).

In the case of single glass, g coincides with τ_e:

g = τ_e

In the case of insulating glass, the solar factor takes into account an additional contribution, q_i, from the radiation reflected and transmitted by the double glazing towards the inside. Thus, the solar factor is equal to:

g = τ_e + q_i

What is the difference between solar factor, thermal transmittance, and light transmittance

The main values in the choice of glass are thermal transmittance, light transmittance, and the solar factor.

The solar factor g is the ratio between the solar energy that manages to pass through the glass entering the environment and the total solar energy that strikes the outer surface of the glazing. It is expressed as a percentage % and is useful for evaluating the energy performance of a glass element.

Thermal transmittance U is the amount of heat that passes from a heated environment to a cold environment through a given surface. It measures the efficiency with which a material resists the passage of heat and primarily answers two questions: how much heat does a building retain and how much does it lose. The unit of measurement for thermal transmittance is W/m²K, Watts per square meter per Kelvin.

The light transmittance (TL) expresses the amount of light transmitted through the glass. It is the percentage of light, visible to our eyes, that the glass of a fixture allows to filter into the internal environment. The higher the % value, the more the room will be naturally lit, saving on the use of artificial light sources.