String inverters operate at high DC voltages, and several modules are wired in series to achieve the voltage range of the inverter. Grid-tied inverter manufacturer Web sites usually include online array string configuration tools to help you figure out what module configurations you can use with each of their inverters. You can plug in various inverter models; PV module make and models; your mounting method (roof, pole, or ground-mount); and the site’s high and low temperatures. The software then shows a range of how many modules are needed in each series string and how many of these series strings can be accommodated in parallel within the inverter specifications.
Microinverter installations, where each module is directly coupled to its own inverter, must use compatible modules. Again, the inverter manufacturer aids by maintaining a list of modules that are compatible with each of their products.
Module Frames & Backsheets. Frames are available in silver and black, and backsheets are either white, black, or clear (for bifacial modules), depending upon the manufacturer and model. If module aesthetics are important to you, you might minimize visual impact on a dark roof by considering modules with black frames and backsheets.
Manufacturing Location. This may be an important consideration for some, since the farther a module has to be shipped, the more “embodied energy” the module has. Additionally, jobs provided here in the United States by PV manufacturing facilities can help stimulate local economies. In an attempt to stimulate the U.S. economy, projects on public buildings that are a part of the federal American Recovery and Reinvestment Act of 2009 (ARRA) must use products that are manufactured in the United States. As a result, some modules (and other balance-of-system components) now carry the “ARRA Compliant” label.
Manufacturers including Evergreen, Schott, Sharp, and SolarWorld all have modules assembled in the United States. However, not every model is assembled here, and cells are often produced out of country. If you want to check if a specific module model or its cells were assembled in the United States, look for the “ARRA-compliant” label and/or contact the module manufacturer.
J-Box, “PV Wire” & Other Installation Considerations. For ground-mounted systems operating at 30 V and above, in which the wiring is “accessible,” raceways are necessary per National Electrical Code article 690.31(A). One way to satisfy this condition is by installing modules with junction boxes, which can accommodate conduit between the modules. There are only a few modules available with junction boxes (Sharp models ND-123UJF and ND-130UJF).
Systems using transformerless inverters, such as from Power One and SMA America, must be “ungrounded.” Per NEC 690.35(D)(3), special “PV Wire” must be used if the module interconnects will be exposed. As a result, many module manufacturers are beginning to use “PV Wire” for their cabling.
For systems installed in hot climates, consider the module’s maximum power temperature coefficient (Pmp temp coefficient: % per °C). The lower the value, the better it will perform at hotter temperatures. The Pmp temperature coefficient is usually about -0.5% per °C. Some modules have values of -0.3% or lower (Sanyo’s HIT modules).
Justine Sanchez is a NABCEP-certified PV installer, Home Power technical editor, and Solar Energy International instructor who is currently pondering all these considerations as she is about to install another PV array at her house.