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.
The Marketing
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.
Mechanical & General
Dimensions
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)].
Area
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.
Thickness
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.
Weight
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
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.
Cell Dimensions
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.
Glazing
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.
Backsheet
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.
Encapsulation
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.
Frame
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.
Connectors
The module lead’s connector type is important. Often called “quick-connects,” many new products are on the market. The old standard—Multi-Contact (MC) 4—has been joined by Tyco, Radox, Amphenol, and others. The 2011 NEC mandates that these connectors be touch-safe and, for circuits greater than 30 volts, require a tool for opening. Most of these connectors are not cross-compatible, so mixing modules will require properly mating connectors, as well as for wire runs to combiner or pass-through boxes.
Factory-installed module leads will be listed in the spec sheet with wire size, insulation type, and length of the leads (positive and negative leads are not always the same length). Wire diameter generally ranges from 14 AWG to 10 AWG; or they may be listed in square millimeters (mm2). For low-voltage systems, less power will be lost to voltage drop if using modules with heavier-gauge wire.
Insulation type on the conductors may be a single listing, such as PV wire, or have multiple cross-listings, including USE-2, RHW-2, XHHW-2, and/or PV wire. All factory-installed module lead insulation types are tested to be sunlight-resistant and flexible at low temperatures, and are heavily or even double-insulated for installation in extreme outdoor environments. However tough these single conductor leads may be, they still must be protected in a raceway when they leave the vicinity of the array.
Junction Box
A junction box is factory-installed on the back of modules for the connections. Many are sealed and inaccessible to the end user. If it is specified as field-serviceable, the junction box can be opened, and leads and bypass diodes can be installed or replaced. For arrays that are readily accessible (for example, a ground-mounted array), field-accessible and conduit-ready junction boxes can allow for fittings and protective raceways to be installed and meet NEC 690.31(A) code requirements for accessible arrays.
Bypass Diodes
Shading a small part of a PV module can have a disproportionally large effect on its output. Additionally, when a module is partially or completely shaded, the current flowing through the module can reverse direction and create hot spots, which can lead to deterioration of the cell, the internal connections, and the module backsheet. A bypass diode stops the reverse flow of current and also directs electrical flow around the shaded section of the module. Nearly all modules come with factory-installed bypass diodes, with the exception of some thin-film modules. A typical 72-cell module with all the cells in series will have three bypass diodes, each bridging a series of 24 cells that can be bypassed if any or all of those cells are shaded. Depending on where they are located on the module and the type of junction box, diodes may be field-accessible. Regardless of the benefit of diodes, shading should be avoided whenever possible.
Modules per Pallet; Pallets per Container
A pallet of modules isn’t a standard quantity. Details on packing information is important to help calculate point loading if pallets are to be placed on a roof, or for staging large job sites.
Electrical Data
I-V Curve
Standard test conditions (STC) are the conditions under which a manufacturer tests modules: 1,000 W per m2 irradiance, 25°C (77°F) cell temperature, and 1.5 air mass index. Real-world operating cell temperature is often 20 to 40ºC above the ambient temperature. STC (bright sun and a relatively low cell temperature) are not typical for field operation of modules, but they do provide a consistent standardized reference to compare modules.
An I-V curve (current-voltage) curve is generated at STC for every cell and module manufactured. The I-V curve contains five significant data points (Pmax, Vmp, Voc, Imp, and Isc; discussed below), which are used for system design, troubleshooting, and module comparisons. I-V curves can also be diagrammed for any operating temperature and irradiance level, but the points listed on a module specification sheet and those printed on the back of the module are at STC unless otherwise stated.
Peak Power (Pmax or Pmp)
The specified maximum wattage of a module, the maximum power point (Pmax), sits at the “knee” of the I-V curve, and represents the product of the maximum power voltage (Vmp) and the maximum power current (Imp). This wattage is produced only under a very specific set of operating conditions, and real environmental conditions (changing irradiance and cell temperature) will alter a module’s Pmax.
Vmp
At STC and tested under load, voltage at max power (Vmp) is the highest operating voltage a module will produce. Vmp, adjusted for highest operating cell temperature, is used to calculate the minimum number of modules in series.
Voc
Open-circuit voltage (Voc) occurs when the module is not connected to a load. No current can flow in an open circuit and, as a result, Voc occurs at the point on the I-V curve where current is zero, and voltage is at its highest (Note: the module produces no power under open-circuit conditions.)
Voc is used to calculate the maximum number of modules in a series string. Because voltage rises as the temperature drops, calculations are performed for the coldest expected operating conditions. This ensures that NEC parameters and equipment voltage limitations are not exceeded.
Imp
At STC, and tested under load, the maximum power current (Imp)is the highest amperage a module can produce. Imp is used in voltage drop calculations when determining wire gauge for PV circuits. This is a design consideration rather than an NEC ampacity calculation, for minimizing voltage drop and maximizing array output.
Isc
Short-circuit current (Isc) is the maximum amperage that the module can produce. There is no voltage when a module is short-circuited, and thus no power. Isc is the measurement used to size conductors and overcurrent protection, with safety factors as required by theNEC.
NOCT
Frequently, nominal operating cell temperature (NOCT) specifications are also listed on a manufacturer’s sheet. These are measurements calculated at different conditions than STC, using a lower sunlight intensity (800 W per m2); an ambient (not cell) temperature of 20ºC; and a wind speed of 1 meter per second; with the module tilted at 45°. The NOCT value itself is the cell temperature—given in degrees Celsius—reached under these conditions, Compared to the STC 25ºC cell temperature, the NOCT value will always be higher, usually by about 20ºC. NOCT values are used to mathematically calculate other test condition data points without resorting to further laboratory tests. NOCT conditions tend to more closely resemble the field conditions PV arrays generally operate in, and so give a perspective on “real-world” module operation.
Other Electrical Parameters
Power Tolerance
Power tolerance is the range within which a module manufacturer is stating the module can deviate from its STC-rated Pmax, and thus what the manufacturer warranty covers. Common values are +/-5%, -0%/+5, and up to +/-10%. A 200-watt module with a +/-5% power tolerance could produce a measured output of 190 to 210 W. Finding modules with a -0% power tolerance can ensure the best value per dollar spent, and keep arrays operating at closer to predicted output.
Module Efficiency & Cell Efficiency
Efficiency is the measure of electrical power output divided by solar input. At STC, power in is equal to 1,000 W per m2 and power out is the rated Pmax point. Assuming a module sized at exactly 1 square meter, and rated at 150 W Pmax, module efficiency would be 150 W per m2 ÷ 1,000 W per m2, which equals 15%. The typical crystalline efficiency range spans 12% to 15%, but there are high-efficiency modules over 19%, and amorphous silicon modules on the low end with efficiencies around 6% or 7%.
Cell efficiencies will be slightly higher than module efficiencies because there is usually a small amount of empty space between cells. When deciding what module to purchase, if W per square meter (known as power density) is the driving factor, then a module with high efficiency should be chosen. But in many instances, there is plenty of room for an array and price per watt will be given higher priority than module efficiency.
Temperature Coefficients
Modules are directly affected by both irradiance and temperature, and because of environmental fluctuations, also experience power output fluctuations. When exposed to full sun, the cells will reach temperatures above the STC temperature of 25°C. And sometimes cell temperatures are lower than 25°C, such as on cold winter days.
Temperature coefficients are used to mathematically determine the power, current, or voltage a module will produce at various temperatures deviating from the STC values.
The temperature coefficient of open-circuit voltage is used to figure out the PV array’s maximum system voltage at a site’s lowest expected temperatures. The temperature coefficient of power can be used along with pyranometer-measured irradiance to calculate the power an array should be producing, which can be compared to actual output to verify proper performance.
Maximum Ratings
Maximum System Voltage
For residential PV systems, the maximum allowed voltage is 600 volts (per NEC 690.7(C)), but ratings on equipment are just as critical to abide by. While most of the equipment—including modules—in PV systems is rated for up to 600 V, they are generally tested to higher voltages, usually twice the listed maximum plus 1,000 V. Maximum system voltage is calculated using the Voc at coldest expected temperatures (see “Back Page Basics” in HP128) so as not to exceed the NEC limit and any limits imposed by the ratings of inverters, disconnects, or conductors. Modules sometimes list a 1,000 V limit, but that is for European installations or engineered commercial and utility-scale systems.
Maximum Series Fuse Rating
This is the maximum current a module is designed to carry through the cells and conductors without damage. While modules themselves are current-limited, excess current can come from other sources (series strings) in parallel, or from other equipment in the system such as some inverters or charge controllers. A fuse or breaker for a series string must be no larger than the maximum series fuse specification.
Design Load
The weight (in lbs. per ft.2, PSF) that a module has been tested to hold without damage. Modules will usually handle 50 PSF. In areas with heavy snow loads, modules with a higher design load should be used and may be required by the permitting authority.
Maximum Wind Speed
This is the maximum wind speed a module can handle without damage, and 120 mph is a common rating. Your local building authority can provide the design wind speed you need to use. In areas with higher-than-normal wind speeds, thin-film or frameless, glass-on-glass modules may be the only choice with a high-enough rating.
Certifications & Qualifications
For a code-compliant installation, modules need to be tested to UL standard 1703, and stamped by a nationally recognized testing laboratory (NRTLs, as listed by OSHA) as meeting this standard. Other NRTLs include CSA, TUV, and Intertek (ETL). Modules often list other compliances and qualifications, including International Standard for Organization (ISO) 9001:2008 which is an international standard for a quality management system.
Fire Safety Class
Plastic-backed modules with glass fronts are nearly always listed as “Fire Safety Class C,” which means they are potentially energized electrical equipment, and no conductive agents (such as water) should be used to fight the fire.
Warranty
Modules list separate workmanship and power warranties. The workmanship warranty is a limited warranty on module materials and quality under normal application, installation, use, and service conditions. Certain parts of modules, including quick connects and some junction boxes, have only short warranties from their manufacturer, and this is reflected in overall workmanship warranties of one to 15 years. Manufacturers may offer replacement or servicing of a defective module under the workmanship warranty.
A limited warranty for module power output based on the minimum peak power rating (STC rating minus power tolerance percentage) means that the manufacturer guarantees the module will provide at least a certain level of power for the specified period of time. Many warranties are stepped—covering a percentage of minimum peak power output within two different time frames. For example, a common warranty guarantees that the module will produce 90% of its rated power for the first 10 years and 80% for the next 10 years. A 200 W module with a power tolerance of +/-5% means that the module should produce at least 171 W (200 W × 0.95 power tolerance × 0.9) under STC for the first 10 years. For the next 10 years, the module should produce at least 152 W (100 W × 0.95 power tolerance × 0.8). Module replacements are frequently done at a prorated value according to how long the module has been in the field. More manufacturers are now offering linear power warranties, which are represented by a maximum percentage power decrease per year for a set number of years, for example, that module power output shall not decrease by more than approximately 0.7% per year after the initial year of service, for the first 25 years.
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Rebekah Hren is a licensed electrical contractor, NABCEP-certified PV installer, and ISPQ-certified PV instructor for Solar Energy International. She lives off-grid and has experience installing and designing PV systems ranging from 10 watts to utility-scale. Rebekah has coauthored two renewable energy books: A Solar Buyer’s Guide and The Carbon-Free Home.
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