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.
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.
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.
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.