PV Purchasing: Page 3 of 3

Top 10 Considerations
Beginner

Inside this Article

Installation of a PV Array
Ready to invest in a PV system? Check out our top 10 tips for choosing the best module.
Siemens Module
This Siemens module is still covered under warranty, even though the module line has twice been transferred, first to Shell Solar and then to SolarWorld.
Ningbo Solar Module
If there’s plenty of room, then lower power-density modules, like this NBsolar 165 (12.9% efficiency), can be less expensive per watt.
SunPower E-19 318-watt Module
At 19.5% efficiency, the SunPower E-19 318-watt module offers one of the highest power densities available.
Module Layout
Module sizes and dimensions need to be carefully considered to maximize roof space, while meeting local fire safety setbacks. The illustrations at left show the importance of comparing different module options. Both rooftops are the same size; both have arrays that use 185 W modules, but the modules have different dimensions. This results in the top left array having 27% more rated capacity. (Note: If modules were arranged in landscape orientation, the bottom array could accommodate 24 modules instead of 22.)
Scheuten Multisol P6-54 Module
Pay close attention to power tolerance ratings: Some modules have both positive and negative ratings; others, like this Scheuten Multisol P6-54, rated at 0 to +10%, have positive-only power tolerances.
Fronius' Online String-Sizing Tool
Online string-sizing tools will determine how many PV modules and strings are needed to meet inverter capacity and voltage range.
Sanyo HIT Modules
Some modules, like Sanyo’s HITs, allow light to pass through the backsheet for underside lighting, as well as power generation from reflected light.
SunPower Modules
Manufacturers now offer more module options—like black frames and backsheets—to suit homeowners’ aesthetic preferences.
SolarWorld Module
Some modules, such as this one from SolarWorld, are made entirely in North America, which means less embodied energy from transportation, as well as creating jobs stateside.
Installation of a PV Array
Siemens Module
Ningbo Solar Module
SunPower E-19 318-watt Module
Module Layout
Scheuten Multisol P6-54 Module
Fronius' Online String-Sizing Tool
Sanyo HIT Modules
SunPower Modules
SolarWorld Module

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).

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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.

Comments (3)

rdrcfo's picture

In regards to installer preference or disfavor with installing PV modules purchased outside does it mean they markup the modules costs as part of the instllation?

Michael Welch's picture

Hi there. Yes, sort of. Like in any other trade, they buy at wholesale and sell at retail to the end consumer. But it is part of the package. If you buy your own equipment, you may not find a reputable installer to put it in. If you do, you are likely to pay a higher installation labor cost for a couple of reasons. First, the seller hasn't made a dime on the equipment, and second because they may not be used to the equipment the customer purchased on their own. But here is the worst of it under this scenario. You probably will not get system support from the installer, your support would need to come from wherever you purchased the equipment, or from the manufacturer if they are willing.

rdrcfo's picture

Thanks Michael your comment was helpful and certainly makes sense.

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