Key Solar Energy Terms: Irradiance, Insolation, TSRF and More

If you work in the solar industry, you know that the amount of solar energy available at a proposed project site is one of the most fundamental factors for determining whether installing solar makes sense for a customer. Obviously, the amount of solar energy—also referred to as irradiance or insolation—where the array will be sited will determine how much energy it can potentially produce.

By-Ranveer Yadav

There are a number of different metrics for expressing the amount of solar energy at a given location, however. Whether you’re new to the industry or are just looking for a refresher on some of the key terms and metrics for expressing how much sunlight will reach your solar array, today’s article has you covered.  

Irradiance maps, like this one above, which visually communicate how much solar energy is available at each point on a roof. (Brighter colors indicate greater irradiance.)

1. Irradiance and Insolation

Irradiance and insolation are perhaps two of the most important terms to know for describing the available solar energy at a project site. The two terms are often used interchangeably in practice within the solar industry, as both quantify the amount of solar radiation a surface receives. However, they measure that value in different ways.

Irradiance is a measure of solar power whereas insolation is a measure of solar energy. Because power refers to the rate of energy transfer over time (not the total amount of energy delivered), another way of thinking of irradiance is that it quantifies the amount of solar energy that arrives in a particular area in a given moment [Watt/m²].

When that value is converted to express the total amount of energy that area receives over a certain interval of time, say one hour, it is communicated in Watt hours (Wh) or, depending on quantity, kilowatt hours (kWh) per unit of area [(k)Wh/m²]. This is a measure of insolation.

2. Solar Access

Solar access is another term used to quantify how much sunlight is available for a solar array at a particular site. It is also referred to as Solar Access Percentage or Solar Access Value. This metric expresses the available solar energy as a percentage of what would be available in perfect (i.e. shade-free) conditions.

Solar access is calculated by dividing the actual solar energy present given shading at the site by the amount of solar energy if there were no shade:

This is a handy metric as it provides an easy way of understanding how significantly shade is reducing the available sunlight. Because even a small amount of shade can disproportionately reduce the power output of a solar array, determining the extent of shading is an important first step in understanding whether a customer’s property—or a particular area of that property—is a feasible location for a solar PV system.

Variety of measures of solar energy including irradiance, solar access, TOF, and TSRF. Moving your cursor over the roof face allows you to see how those values vary at different points.

3. Tilt and Orientation Factor (TOF)

Tilt and Orientation Factor, or TOF, is a metric that takes into account how the slope and direction of a given surface impact the solar energy that surface receives. The specific tilt and orientation that maximize the solar energy reaching a surface varies depending on latitude. Like solar access, TOF is a percentage that expresses actual conditions compared to optimal conditions.

TOF is calculated by dividing the solar energy available at the actual tilt and orientation of the surface, by what would be available at the optimal tilt and orientation.

4. Total Solar Resource Fraction (TSRF)

Total Solar Resource Fraction, or TSRF, is a measure of available solar energy that takes into account the two other metrics we’ve discussed in this article—solar access and TOF. TSRF can be calculated by multiplying the solar access of the site by the TOF percentage.

TSRF provides a more complete picture of how much solar energy will be available for the solar panels to convert into electricity. This is because it takes into account both the percentage of available solar energy at a site given shading (solar access) and how much of that energy will reach the surface where solar panels will be mounted given its tilt and orientation (TOF).

Of course, there are other factors—like the stringing configuration and the capabilities of the components you use—that will significantly impact the amount of energy your solar design will produce given the sunlight it receives. However, accurately quantifying the solar energy that is available to the PV system is an essential first step in correctly estimating the system’s energy output. These values once had to be determined based on manual measurements at the project site. Today, however, advanced solar software applications can accurately calculate these values without a site visit—saving installers time and money and reducing the potential for error.