Solar Energy Fundamentals & PV Cell Science
Irradiance, Standard Test Conditions, temperature coefficients, and environmental factors that drive PV module performance.
- Define irradiance and insolation and explain how they affect PV output
- State the three Standard Test Conditions (STC) values every installer must know
- Explain how cell temperature affects voltage, current, and power output
- Describe the impact of soiling, altitude, and wind on module performance
- Apply temperature coefficients to estimate real-world power changes
Lesson 1
Irradiance, Insolation & the Solar Resource
What Is Irradiance?
Irradiance is the measure of solar power striking a surface at any given instant. It is measured in watts per square meter (W/m^2). Think of irradiance as the "speedometer" of sunlight - it tells you how intense the solar energy is right now.
On a clear day near solar noon, irradiance at ground level typically reaches around 1000 W/m^2. On overcast days, it may drop to 100-300 W/m^2. At sunrise and sunset, irradiance is near zero because sunlight travels through more atmosphere.
Irradiance vs. Insolation
While irradiance measures instantaneous power, insolation measures the total energy received over a period of time - typically one day. Insolation is expressed in kWh/m^2/day (sometimes called "peak sun hours"). A location with 5 kWh/m^2/day of insolation receives the equivalent of 5 hours at full 1000 W/m^2 irradiance each day.
Current output from a PV module is directly proportional to irradiance. When irradiance doubles, current roughly doubles. This is the single most important relationship for understanding how much electricity a solar array produces.
Air Mass and Atmospheric Effects
The term Air Mass (AM) describes how much atmosphere sunlight passes through. AM1.0 means sunlight is coming straight down (sun directly overhead). AM1.5 is the standard reference - it represents sunlight passing through 1.5 atmospheres, which occurs when the sun is about 48 degrees above the horizon. This is the average condition for temperate latitudes.
The NABCEP exam references AM1.5 as part of Standard Test Conditions. It is not an arbitrary number - it represents a realistic average of how much atmosphere sunlight passes through across the continental United States.
Irradiance is measured in W/m^2 and drives module current output. When the exam asks what unit measures solar intensity at a surface, the answer is always W/m^2.