Table of Contents
Understanding Solar PV Module Ratings: What You Need to Know
1. PV Module Rating
Solar PV Module Rating – In the solar industry, the peak power rating of a panel is frequently abbreviated as kWp. kWp is the peak power of a PV module or system that describes the energy output of a system achieved under full solar radiation under set Standard Test Conditions (STC). Solar radiation of 1,000 W/m2, module temperature of 25°C, and solar spectrum air mass of 1.5 are used to define standard conditions. This is generally referred to as a “full sun” condition. That is full irradiance.
Less than full sun will reduce the current output of the cell by a proportional amount. For example, if only one-half of the sun’s energy (500 W/m2) is available, the amount of output current is roughly cut in half because the solar cell only has half the brightness to generate electricity.
STC conditions are:
- 1,000W/m2 of sunlight (solar irradiance, often referred to as comparable to clear summer noon time intensity).
- 25°C cell temperature.
- Spectrum at air mass of 1.5 — (solar spectrum as filtered thickness of the atmosphere (ASTM Standard Spectrum).
A manufacturer may rate a particular solar module output at 100 Watts of power under STC and call the product a “100-watt solar module.” This module will often have a production tolerance of +/-5% of the rating, which means that the module can produce 95 Watts and still be called a “100-watt module.” To be conservative, it is best to use the low end of the power output spectrum as a starting point (95 Watts for a 100-watt module).
1.1 Module Efficiency
The efficiency of each PV product is specified by the manufacturer. Efficiencies range from as low as 5% to as high as 15%–19%. A technology’s conversion efficiency rate determines the amount of electricity that a commercial PV product can produce. For example, although thin-film amorphous silicon PV modules require less semiconductor material and can be less expensive to manufacture than crystalline silicon modules, they also have lower conversion efficiency rates.
These will need close to twice the space of a crystalline silicon PV array because its module efficiency is halved, for the same nominal capacity under Standard Test Conditions (STC).
The conversion efficiency of different PV cell technologies is summarized in the Table below.
Conversion efficiencies of various PV module technologies
| PV type | Description (Color and texture) | Module efficiency | Surface area for 1kWp system (m2) |
|---|---|---|---|
| Monocrystalline (m-Si) | Blue, grey, black, high light absorption | 14-19% | 7 |
| Polycrystalline (p-Si) | Bright, bluish, speckled tone | 12-15% | 9 |
| Thin film Amorphous silicon (a-Si) | Reddish-black, very flexible/durable | 6-8% | 17 |
| Thin film CIGS/ CIS (Copper Indium Gallium Selenide/Copper Indium Selenide) | Black, shiny cell – flexible or rigid | 9-12% | 11 |
| Thin film CdTe (Cadmium telluride) | Grey-green rigid cell | 7-10% | 14 |
| Titanium dioxide (TiO2) dye | Light brown translucent window system | 3-5% | 20 |
Notes:
Each technology has an associated range of output in watts per square foot or per square meter and cost per watt. For example:
- PV Modules with higher efficiency will have a lower surface area for equivalent watts. Installation and racking costs will be less with more efficient modules, but this must be weighed against the higher cost.
- Crystalline silicon panels have higher electricity outputs per square meter, but greater costs and design constraints. The power output of single-crystalline and polycrystalline modules is almost similar.
- Thin film amorphous silicon modules have lower rated efficiencies than crystalline silicon modules, but these are less expensive and may be integrated more easily onto the irregular surfaces. Data also suggests that in cloudy weather, all thin film types tend to perform better than crystalline silicon.
- The capacity of the PV system is physically limited to the dimensions of the building’s available surface area. The balance between the amount of power required and the amount of surface area available can determine the type of PV technology that will be used.
2. PV System Components
The key parts of a solar PV energy generation system are:
- Photovoltaic array to collect sunlight
- An inverter to transform direct current (DC) to alternating current (AC)
- A set of batteries and a charge controller for stand-alone PV systems
- Other system components.
All system components, excluding the PV modules, are referred to as the balance of system (BOS) components.
2.1 PV Array
Solar PV Module Rating – A PV Array is made up of PV modules, which are environmentally sealed collections of PV Cells— the devices that convert sunlight to electricity. The most common PV module is 5 to 25 square feet in size and weighs about 3-4 lbs/ft2. Often, sets of four or more smaller modules are framed or attached by struts in what is called a panel. This panel is typically around 20-35 square feet in area for ease of handling on a roof. This allows some assembly and wiring functions to be done on the ground if called for by the installation instructions.
2.2 Batteries
Battery stores electric power for operation during nighttime or during extended periods of cloudy or overcast weather when the PV array by itself cannot supply enough power. The number of days the battery storage capacity is available to operate the electrical loads directly from the battery, without any energy input from the PV array, is called the days of “autonomy” in a standalone PV system.
For common, less critical PV applications, autonomy periods are typically designed for between two and six days. For critical applications involving essential loads or public safety, autonomy periods may be greater than ten days. Lead-acid or lithium-ion batteries are typically used.
2.3 Inverter
The photovoltaic array and battery produce DC current and voltage. The purpose of an inverter is to convert the DC electricity to AC electricity used by your electrical appliances and/or exportable to the AC grid. The typical low voltage (LV) supply into a residential or small commercial building will be either 120V AC single phase or 408V AC three phase.
Inverters are offered in a wide range of power classes, ranging from a few hundred watts (normally for stand-alone systems), to several kW (the most frequently used range) and even up to 2,000 kW central inverters for large-scale systems.
2.4 Charge Controller
Batteries are connected to the PV array via a charge controller. The charge controller protects the battery from overcharging or discharging. It can also provide information about the state of the system or enable metering and payment for the electricity used.
2.5 Balance of System (BOS)
In addition to the Solar PV Module Rating, battery, inverter, and charge controller, there are other components required in a solar PV microgrid system; these components are referred to as Balance of Systems (BoS) equipment. The most common components are mounting structures, tracking systems, electricity meters, cables, power optimizers, protection devices, transformers, combiner boxes, switches, etc.
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References:
A. Bhatia, Course No: R08-002, https://www.cedengineering.com
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