The amount of solar energy striking a square meter of surface varies dramatically depending on location and time. To understand this energy, two specific terms define solar radiation. Solar irradiance refers to the instantaneous power density received from the sun, measured in watts per square meter (\(\text{W/m}^2\)). This figure provides a “snapshot” of the sun’s power at any given moment.
Solar insolation, conversely, is a cumulative measure representing the total energy received over a specified period, typically a day or a year. This metric is most often expressed in kilowatt-hours per square meter (\(\text{kWh/m}^2\)) per day. Insolation provides a more practical value for energy generation calculations than instantaneous irradiance.
The Theoretical Maximum
The absolute maximum solar energy available is measured outside of the Earth’s atmosphere, perpendicular to the sun’s rays. This value is known as the solar constant, which is approximately 1,361 \(\text{W/m}^2\). This figure represents the total solar irradiance before any atmospheric interference occurs.
The solar constant is not an unchanging physical constant, but rather a long-term average that fluctuates slightly. Variations occur due to the Earth’s elliptical orbit around the sun, which can cause the value to change by about \(\pm 3\%\) annually. Minor changes in the sun’s own output, related to the 11-year sunspot cycle, also cause fluctuations of about \(0.1\%\).
This theoretical maximum is not achievable on the Earth’s surface because of the atmosphere. The value serves as a reference point, indicating the maximum potential energy input before any losses are factored in. It is not a practical value for ground-based solar energy systems.
Energy Reaching the Ground
The energy value at the Earth’s surface is drastically reduced compared to the theoretical maximum due to atmospheric effects. As solar radiation passes through the atmosphere, a significant portion is reflected, scattered, and absorbed by air molecules, clouds, dust, and water vapor. This process typically reduces the instantaneous power to a range between approximately \(100 \text{W/m}^2\) and \(1,000 \text{W/m}^2\) at any given moment.
The practical measure for solar energy that reaches the ground is called Global Horizontal Irradiance (GHI). GHI includes both the direct sunlight and the diffuse light scattered by the atmosphere. Under clear skies at solar noon near the equator, the GHI can reach the maximum ground-level value of about \(1,000 \text{W/m}^2\). This \(1,000 \text{W/m}^2\) figure, often called “one sun” or “peak sun,” is a common benchmark in the solar industry.
To calculate the total available energy over time, engineers use the concept of Peak Sun Hours (PSH). PSH is numerically identical to the average daily solar insolation and is expressed in \(\text{kWh/m}^2/\text{day}\). This metric is the most practical figure for system sizing. Typical daily PSH values globally range from 3 to 7 \(\text{kWh/m}^2/\text{day}\), with the specific PSH for any location determined by its latitude, local weather patterns, cloud cover, and the time of year.
Panel Efficiency and Usable Output
Solar panels convert only a fraction of the incoming solar energy into usable electrical power. The efficiency of modern commercial panels typically falls between \(15\%\) and \(25\%\). This means that for every \(1,000 \text{W/m}^2\) of solar power hitting the panel, only \(150 \text{W/m}^2\) to \(250 \text{W/m}^2\) is converted into electricity.
Panel efficiency is determined by tests conducted under strict laboratory parameters known as Standard Test Conditions (STC). STC requires an irradiance of \(1,000 \text{W/m}^2\) and a cell temperature of \(25^{\circ}\text{C}\). Manufacturers use these fixed conditions to calculate the panel’s nameplate wattage rating, such as a \(400 \text{W}\) panel.
The STC rating provides a basis for comparison, but it does not represent the panel’s average real-world output. The actual electrical output is measured in Watts (\(\text{W}\)) or Kilowatts (\(\text{kW}\)). This output is determined by multiplying the panel’s area by the incoming irradiance and its efficiency.
Real-World Factors Affecting Capture
Beyond the panel’s inherent efficiency, several environmental and site-specific factors further reduce the power captured in a real-world installation. One significant factor is the tilt and angle of the solar array relative to the sun’s path. Optimizing the panel angle to face the sun directly is necessary to maximize energy capture.
A major cause of power loss is temperature derating, as solar panels operate less efficiently when they get hot. The maximum power output is measured at \(25^{\circ}\text{C}\), but a panel’s operating temperature on a hot, sunny day can easily exceed \(60^{\circ}\text{C}\). For every degree Celsius above the \(25^{\circ}\text{C}\) benchmark, a panel loses power, typically at a rate of \(0.3\%\) to \(0.5\%\) per degree. This can result in a \(10\%\) to \(15\%\) drop in power output compared to the panel’s STC rating.
Another significant loss comes from shading and soiling, which includes dust, bird droppings, and leaves accumulating on the panel surface. Even partial shading of a single cell can disproportionately reduce the output of the entire panel or string. Minor losses also occur within the system components, such as the wiring, cables, and the inverter that converts the DC power to AC power.