Definition: A company that designs and builds a line of PV modules.
Importance: PV manufacturers include global energy companies like BP and GE. Manufacturers with long histories of producing consumer or industrial electronics such as Sanyo, Sharp, and Kyocera are devoting significant resources to PV production capacity. And there are "pure play" companies that focus on one thing—manufacturing photovoltaic modules. Examples include Advent Solar, Canadian Solar, Day4Energy, Evergreen, SolarWorld, and Suntech Power.
Definition: The identifier used to distinguish one module from another.
Importance: Other than giving you a reference point to compare modules, model specifications and availability often change and should be verified prior to purchasing.
Rated Power at STC (watts)_
Definition: Module wattage rating at standard test conditions (STC)—1,000 watts per square meter solar irradiance, 25°C (77°F) cell temperature.
Importance: The STC rating establishes a consistent basis for comparing the power output of individual PV models— but this specification shouldn't be mistaken for the actual power a module will generate consistently in the field. Rated power tolerance (described below) and array operating temperatures are two factors that result in real-world module output that, in most instances, will be significantly lower than a module's rating at STC. If you purchased a 100-watt module with a measured power tolerance of minus 3%, that module could potentially generate 97 watts in the field. If the module was installed in a hot climate like Southern California and the array spent much of its life operating at 50°C (122°F), the actual output of that 100-watt module during the heat of the day might be about 85 watts. The real-world wattage would be better if the module had a 0% power tolerance rating and was operating in a cooler climate, and worse if it had a lower tolerance rating and was installed in a hotter climate.
Rated Power Tolerance (%
Definition: The specified range within which a module will either overperform or underperform its rated power at STC.
Importance: Power tolerance is the most contentious module specification. Depending on the module, this specification can vary from as much as plus 10% to minus 9%. With only a positive power tolerance (plus 2.5%), Evergreen's new 195-watt module is guaranteed to generate at least 195 watts at STC. Shuco also manufactures two modules with no negative power tolerance. Due to the recent trend of rating modules in small increments, for example, a 5-watt difference between models, the reality is that modules that meet the power tolerance of the next highest model will be classified as such. The result? Modules are more likely to produce at the lower end of the tolerance range. The bottom line is that the tighter the rated power tolerance, the better, so you can be assured that you're getting the wattage you pay for.
Rated Power per Square Foot (watts)_
Definition: Power output at STC per square foot of module (not cell) area; calculated by dividing module rated power by the module's area in square feet.
Importance: If you have limited space available for a PV array, this metric will help you determine which module will maximize power output in a given area (power density). Rated power per square foot is one tangible way to compare the efficiency of one module to another. Currently, specific modules manufactured by Sanyo and SunPower achieve the highest power densities.
Definition: The ratio of output power to input power, or how efficiently a PV module uses the photons in sunlight to generate DC electricity.
Importance: Module efficiency is another indicator of which modules will generate the highest power if space is limited. While high efficiency is great, it typically comes at an increased cost. For the modules surveyed, efficiencies range from 10.3% to 19.3%. Manufacturers may also advertise the efficiency of individual cells, which should not be confused with overall module efficiency—a more important figure to consider. Finally, there has been a fair amount of hype on the Internet recently about solar technologies reaching efficiencies greater than 40% in the lab. But these devices are not ready for prime time, and probably won't be for decades. Most importantly, they shouldn't be compared to warranted, commercially available modules that you can put to work today.
Module Physical Dimensions (inches)_
Definition: Length, width, and depth of a given module.
Importance: Module dimensions vary, often significantly. Careful consideration of module dimensions during the system design phase will result in an attractive array that is visually integrated with the building and uses available space wisely. Poor layout planning can result in an installation that's less aesthetically pleasing, such as arrays extending past the roof's ridgeline or gable ends.
Definition: Module weight in pounds.
Importance: The total weight of an installed array (including modules and racking) is not usually a factor that needs to be considered unless ballasted mounts will be used or engineering is required for the project. The weight figures here are for modules only and do not include packaging for shipping.
Series Fuse Rating (amps)
Definition: Amperage value of a series fuse used to protect a module from overcurrent, under fault conditions.
Importance: Series strings of modules wired in parallel at a combiner box typically require overcurrent protection for each string. The module manufacturer specifies the amperage rating of the required fuse or breaker. Many batteryless inverters are designed to accept the individual output wiring of two or more series strings without additional series fusing.
Definition: Module configuration.
output terminal or cable/connector
Importance: To decrease installation time, most PV manufacturers have moved away from accessible junction boxes where installers terminated module wiring at screw-type connectors. Preinstalled cabling that includes "plug and play" weather-tight connectors is now the standard. The most common connector types are manufactured by Multi-Contact USA, which offers a line of connectors commonly referred to as MC connectors. Two manufacturers, Day4Energy and GE, use Solarlok connectors manufactured by Tyco Electronics.
Materials Warranty (years)
Definition: A limited warranty on module materials and workmanship under normal application, installation, use, and service conditions.
Importance: Of the modules surveyed, materials warranties vary from 1 to 10 years.
Power Warranty (years)_
Definition: A limited warranty for module power output based on the minimum peak power rating (STC rating minus power tolerance percentage) of a given module.
Importance: Few consumer products have warranties that come anywhere close to those carried by PV modules: at least 20 years. The fine print typically breaks down module power warranties based on a percentage of minimum peak power output within two different time frames— 90% of minimum peak power is typically guaranteed for 10 years, and 80% for 20 to 25 years.
Definition: The material that comprises a specific cell, based on the cell manufacturing process.
Importance: There are three general types of PV cell materials— monocrystalline, polycrystalline, and thin film. As the specs here indicate, neither mono- nor polycrystalline cells show a clear performance advantage. A module's performance is more directly related to the specifications of the particular cells used, and the specific design of a given module. Thin-film modules have roughly half the power density of crystalline module types, and other than a couple of companies that combine two smaller thin-film modules together, no framed thin-film modules rated at more than 100 watts at STC are available in the United States. UniSolar manufactures flexible roof laminates that are adhered to standing-seam metal roofing. Depending on their length, they may generate more than 100 watts per laminate. An interesting note is that Sanyo manufactures modules that combine monocrystalline cells with layers of thin-film material, which enables the modules to use a wider range of the sun's light spectrum.
Cell Size (inches)
Definition: Indication of relative cell size.
Importance: The voltage of PV cells is relatively consistent no matter what their size, while cell current directly correlates to cell area. Roofs with limited space may benefit from modules with smaller cells to increase series string voltage to match a specific inverter. Modules with larger cells are well suited for high-power commercial installations.
Cells in Series
Definition: Number of individual PV cells wired in series to generate the module design voltage.
Importance: Module voltage increases as additional cells are wired in series. Historically, module design voltage was based on recharging a battery bank of a specific voltage (typically multiples of 12 volts nominal). Today, most PV systems operate at high voltages (up to 600 VDC), are grid connected, and use inverters and charge controllers that optimize array output over a wide voltage range. As a result, some modules have a maximum power voltage (based on the number of cells in series) that will not be compatible with systems using non-maximum power point tracking (MPPT) charge controllers.
Cells in Series per Bypass Diode_
Definition: Bypass diodes provide an alternate path for electricity to flow if a portion of a module is shaded. A certain number of cells in series are configured with bypass diodes wired in parallel between series strings.
Importance: Poorly designed arrays may operate near the bottom of the voltage-tracking window of a batteryless inverter. In this instance, module shading can cause the array voltage to drop below the minimum inverter voltage threshold, and power output will cease until the array is again sufficiently illuminated. Bypass diodes allow nonshaded cell series strings within a module to continue to generate electricity if another series string within the same module is shaded, keeping the array voltage as high as possible to keep the system functioning.
Maximum Power Voltage (Vmp)_
Definition: The voltage generated by a PV module or array when exposed to sunlight and connected to a load—typically a batteryless inverter or a charge controller and battery.
Importance: Batteryless inverters have a range in which they track and optimize the output of a PV array as its voltage and current vary throughout the day. The maximum power voltage of an array should be designed to stay within the tracking window of your inverter or MPPT charge controller.
Rated Power at STC (W)
Rated Power Per Sq. Ft. (W)
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