Specification sheets are readily available from manufacturers and distributors for the thousands of PV modules on the market today. Spec sheets—or cut sheets—serve as marketing material for the manufacturers, but also contain a large amount of technical information necessary for PV system design—and for choosing which module serves an application best.
A module spec sheet needs to be thoroughly investigated to ensure compatibility before purchasing any PV system equipment. Once a module has been chosen, be sure to follow the complete manufacturer’s installation instructions, which are separate from the spec sheet. This article defines and explains the pertinent technical data listed on a spec sheet.
Spec sheets usually start off with a few glossy pictures and advertising about the module’s quality, efficiency, or a special design aspect. This is just marketing, surrounding the technical data the spec sheet delivers. Not every spec sheet follows the same layout, but at minimum, the sheets contain electrical and mechanical data.
Given in inches and/or millimeters, a module’s size determines how many can fit in a given space, whether on a roof or on a ground- or pole-mount. If rack information is also known, the number of rows and each row length can be determined, based on the space intermodule clips add between modules (typically 1/2- to 1-inch per gap). Many manufacturers will also diagram the appropriate rail positioning for their modules, such as how much of the module can overhang the rails, and whether rails can cross the module in a landscape or portrait orientation or both. Be sure to follow the complete manufacturer’s instructions, as required by the National Electrical Code [110.3(B)].
Simply width times length, the area of a module is useful for checking power density (watts per ft.2). The total module area can be used along with site-specific data to calculate wind uplift forces and thus lag bolt requirements, or to calculate weight loading on a structure.
The frame thickness determines what rack components to use, like slip-in racks, or the required size of end and intermodule clips. Typically, thicker frames result in sturdier, although heavier, modules.
Most permitting authorities will ask for basic structural engineering data for roof-mounted PV arrays, and there will be a limit to the weight that can be added to a roof structure. Module weight, rack weight, and engineering data will restrict the quantity of PV modules that can be installed. Crystalline, glazed modules with plastic backsheets typically weigh about 3 pounds per square foot.
Many jurisdictions allow PV modules to be installed on pitched residential roofs without a professionally engineered design, as long as there is only one layer of existing roofing material present.
Cells will be either monocrystalline, polycrystalline, ribbon silicon, thin-film, or even multiple silicon layers, such as with Sanyo’s HIT module. Electrical characteristics, efficiencies, and appearance vary by cell type (see “A Peek Inside PV,” HP132).
Modules can have variable numbers of cells (usually between 36 and 108), with each crystalline cell operating at around 0.5 VDC, wired in series or series-parallel configurations. For example, a 72-cell module with all cells in series will operate at a voltage of about 36 volts. But a 72-cell module with two series strings of 36 cells paralleled will operate at about 18 V, perfect for charging a 12 V battery.
While all crystalline PV cells operate near 0.5 volts per cell, the diameter of the cell (normally 5 or 6 inches) will partially determine the current output of the cell, with larger cells producing higher current.
Most crystalline modules use low-iron, high-transparency tempered glass with an antireflection surface treatment. Low-iron glass has high clarity, and tempered glass shatters into small fragments, instead of sharp shards, if broken. Modules are strenuously tested for weight loading and impact resistance, and the front glazing of a module is extremely durable. Thin-film modules may use a polymer film (plastic) as the front sheet, which is designed for arrays in high-impact environments.
Most crystalline modules have a plastic backing material that seals the cells against environmental infiltration. The most common material is Tedlar, a polyvinyl fluoride film. This backsheet is the fragile underbelly of the module, and care must be taken not to scratch it.
Some crystalline modules have a glass backing (such as bifacial modules that can also utilize light reflected to the back side). Thin-film modules have a wider range of backings, including glass, stainless steel, and varieties of tough plastic polymers.
A glue laminate, such as ethylene vinyl acetate, is used to seal and protect the back and front of cells within the module glazing and backsheet.
Some crystalline modules are frameless (Lumos Solar; Silicon Energy), with a glass front and back, similar to the technique used for many thin-film modules. But most crystalline modules have anodized aluminum frames, with clear-coated aluminum and black being the most commonly available colors. Noting the frame information can help with other decisions, for example making sure that the color of the frame matches rack and clips, and to help blend with the roof color.