Photovoltaic (PV) Panel Characteristics: A Comprehensive Guide

Photovoltaic (PV) panels are pieces of electrical equipment with distinct characteristics defined by the materials used in their construction. These characteristics describe the voltage and current behavior of the panels, typically illustrated by a graph known as the “knee curve”:

This knee curve above graphically represents four key parameters used to describe any PV panel, and these are typically listed on datasheets and website product descriptions.

V_oc – Open Circuit Voltage

This is the maximum voltage a PV panel can produce under specified test conditions (explained later). It indicates the contribution each panel can make to the voltage of a string of panels. It is essential for ensuring that the connected inverter can handle the total voltage.

V_mpp – Voltage at Maximum Power Point

This represents the voltage at which the PV panel delivers its maximum power. Inverters, specifically Maximum Power Point Trackers (MPPT), aim to maintain this level to maximize power output. Unlike V_oc, V_mpp is not critical for sizing but indicates typical operating voltage.

I_sc – Short Circuit Current

This is the maximum current the PV panel can produce when the circuit is closed. It helps ensure the current levels stay within safe boundaries for the wiring, isolators, and inverters.

I_mpp – Current at Maximum Power Point

This describes the current level at the maximum power point, indicating the expected current output during typical operations.

Standard Test Conditions (STC)

The above characteristics are determined at Standard Test Conditions (STC), which are as follows:

• Cell Temperature: 25°C
• Solar Irradiance: 1000 W/m²
• Mass of Air: 1.5

These conditions allow for consistency when comparing different PV panels. However, it’s essential to remember that real-world conditions vary, and temperature changes can affect these parameters.

Temperature Variations

Temperature fluctuations can significantly impact PV panel characteristics, particularly V_oc and I_sc. For example, solar panels installed in colder climates where temperatures can drop to -10°C, might behave differently from those tested at 25°C. Thus, it’s crucial to consider these variations during system design and ensure that the entire system remains within safe operating limits. You must ensure the system components can handle the expected variations. Some solar manufacturers often provide temperature coefficients in datasheets to help calculate expected values at different temperatures.

Normal Operating Cell Temperature (NOCT)

Some solar manufacturers may also provide Normal Operating Cell Temperature (NOCT) values to give a more realistic estimate of PV panel performance in real-world conditions. NOCT typically considers:

• Irradiance on Cell Surface: 800 W/m²
• Air Temperature: 20°C
• Wind Velocity: 1 m/s
• Mounting: Open backside (for ventilation).

Since NOCT reflects conditions closer to real-world scenarios, it can be a valuable guide for comparing panel performance in actual installations. Understanding the performance characteristics of photovoltaic (PV) panels is crucial for making informed decisions when comparing different solar panels. A key question that often arises when selecting a solar panel is: “Will this solar panel produce more energy than another solar panel at this particular site?” This question is far more practical and context-driven than asking, “Will this solar panel output more if its solar cells are maintained at 25°C?”. However, because NOCT values are typically lower than STC values, they are less commonly used for sizing purposes.

In summary, these electrical characteristics are crucial for understanding PV panel performance and ensuring that your solar energy system is designed safely and efficiently. Always consider both STC and NOCT values when evaluating PV panels to ensure your system performs as expected under various conditions.

---

We have more about renewable energy on our solar blog.