While photovoltaic (PV) modules are among the simplest electrical devices in a solar system compared to inverters or switchgear, their technical parameters hold deeper significance than often realized. This article provides a comprehensive breakdown of key PV module specifications:
1. PV Module Power (Wp)
Definition: Maximum output power under Standard Test Conditions (STC): Irradiance 1000 W/m², Cell Temperature 25°C, Air Mass AM1.5.
Key Insights:
Nameplate power represents peak STC performance. Actual operating power varies significantly with weather, load, and environmental factors.
Use "Wp" (Watt-peak) exclusively for STC-rated power. Use "W" (Watts) for real-time output.
Example: "A 550Wp module operating at 300W" means its peak capacity is 550Wp, but its current output is 300W. These units are not interchangeable.
2. Conversion Efficiency (%)
Definition: Ratio of maximum output power (Wp) to total solar power incident on the module surface under STC, expressed as a percentage.
Key Insights:
Calculation Example: A 580Wp module (2279 x 1134 mm) has an incident light power of 2.279m x 1.134m x 1000W/m² = 2584.386W. Efficiency = 580W / 2584.386W x 100% ≈ 22.44%.
Real-world efficiency is dynamic, influenced by irradiance, temperature, and load.
Efficiency typically decreases under lower irradiance, with the rate of decline varying by cell technology and manufacturing quality.
3. Low-Light Performance
Definition: Characterizes module behavior (efficiency, power output, voltage/current) under sub-optimal light conditions (dawn, dusk, cloudy/rainy weather).
Key Insights:
Measured at irradiance levels below 200 W/m² (significantly lower than STC), it's critical for assessing performance in diverse climates.
Influenced by cell material, processing, and encapsulation technologies. Modern modules exhibit vastly improved low-light response.
Directly impacts total daily energy yield. Modules with superior low-light performance generate more energy in high-latitude regions or areas with frequent cloud cover.
4. PV Modules as Current Sources
Definition: Output current remains relatively constant under fixed irradiance, while output voltage adjusts according to the connected load.
Key Insights:
Vmp (Voltage at Maximum Power Point): The voltage output at peak power under STC. Actual operating voltage fluctuates between 0V and Voc (Open-Circuit Voltage).
Inverters/controllers use Maximum Power Point Tracking (MPPT) to dynamically adjust the load, forcing the module to operate at Vmp for optimal power extraction.
MPPT Operation Sequence: The controller detects Voc upon connection. Once module voltage exceeds the MPPT start voltage, the algorithm engages to stabilize operation at Vmp.
5. Fill Factor (FF %)
Definition: Ratio of actual maximum power (Pm) to the theoretical maximum power (Voc × Isc) under STC, expressed as a percentage (FF = Pm / (Voc × Isc)).
Key Insights:
Voc × Isc represents the theoretical maximum power limit dictated by the cell's inherent photoelectric properties.
FF quantifies how effectively a module converts this theoretical potential into usable power (Pm).
High-quality modules typically exhibit FF values between 70% and 85%. FF is sensitive to cell material, manufacturing processes, and temperature.
6. Bifaciality Ratio
Definition: Ratio of rear-side power generation to front-side power generation under STC (Bifaciality Ratio = Prear / Pfront %).
Key Insights:
Example Gains: Desert (28% albedo): +5.6% avg gain vs. monofacial; Commercial flat roof (12.8% albedo): +3.8% avg gain.
TOPCon modules lead in bifacial performance, with mass-produced bifaciality ratios typically >80%. Example: JinkoSolar's Tiger Neo 3.0 achieves 85%.
Tongwei's TOPCon-based ultra-high bifaciality module, certified by TÜV Rheinland, set records with a cell bifaciality of 94.3% and module bifaciality of 91.7%.
Bifaciality Ratio ≠ Energy Gain: Actual rear-side energy gain depends on installation factors: ground albedo (reflectance), mounting height, row spacing, tilt angle, and site conditions.
