Whether you are a homeowner wanting to install your own PV system or a professional installer, one challenge will be choosing, among hundreds of options, which module will be the best fit.

Selecting modules requires an understanding of module attributes and specifications. Once you are familiar with those, the job becomes ranking modules based on your criteria. You will find that the most important module characteristics depend on the site and your system goals.

This article discusses the top module considerations: module efficiency, price, aesthetics, reliability, manufacturing location, and integrated features. Then, we consider some common scenarios and how module attributes fit them.

Module Characteristics

Efficiency

Module efficiency is a function of power output per square foot (W/ft.2). The higher this value, the more energy a specific footprint can produce. Module outputs range from about 11 to 19 watts per square foot. For example, let’s say the usable shade-free area on your rooftop measures 240 square feet (20 by 12 feet). Using modules that produce 15 W per square foot yields a 3.6 kW array.

Module dimensions also come into play when determining actual array wattage. Common 60-cell modules are about 65 by 39 inches wide. For our example, we could fit two rows of six modules in a portrait configuration. If the module wattage is 265 watts (with an efficiency of about 15 W/ft.2), our array will be 3.18 kW.

Price

Module price is a factor for most folks. The good news is that module prices have dropped dramatically over the last few years. In 2008, modules were approximately $4 per watt; it is now common to find them for less than $1 per watt. Total system costs followed the same trend—in 2008, they hovered around $8 per watt; now, the average cost is less than $4 per watt (before incentives; assuming professional installation).

Once you factor in any local incentives and the 30% uncapped residential federal tax credit (currently set to expire at the end of 2016), it is no wonder we are seeing so much growth in the PV industry. In fact, in the first quarter of 2014, 74% of new electrical generation in the United States came from solar. Residential PV installations exceeded commercial installations for the first time since 2002, and more than one-third of the residential PV systems came online without any state incentives.

Style

Aesthetic considerations vary by the homeowner, but can be important to many. Options are available to help modules blend with roofing materials. While the standard look has been aluminum/silver frames with a white back-sheet, both frames and back-sheets can be specified in black, and module racks can be ordered to match.

Some specialty PV modules have clear back-sheets. Used in PV awnings, they provide some filtered light underneath the array. Additionally, there are frameless modules (some also with clear back-sheets) that provide a uniform, modern look to the array.

Reliability

Reliability reflects how a module will stand the test of time. Will it actually produce its rated power and be warranted for years to come? This is important, especially considering the rate in which module prices have fallen over the last few years. Have manufacturers been forced to cut corners in the manufacturing process to stay competitive? And will those cost-cutting measures haunt us in the future?

In a perfect world, there would be third-party-verified reliability—but such a certification system doesn’t exist yet. However, various stakeholders in the PV industry are exerting some pressure, resulting in several PV testing labs now offering comparative accelerated testing programs and proposals for quality assurance in manufacturing.

In the meantime, we can buy modules from long-term manufacturers with a good track record. Ask your module manufacturer to share its warranty return rates. Some manufacturers will provide data from independent testing facilities that document field performance. Finally, ask other end users about their experiences with that particular manufacturer.

One way to keep tabs on your modules’ performance is to use module-level monitoring through microinverters or DC optimizers. They can provide real-time side-by-side comparison, making module performance problems easier to identify. This can be useful should you need to file a warranty claim.

U.S.-Made

Manufacturing location is important if you prefer U.S.-made products, and it is critical for federal projects that must comply with the American Recovery & Reinvestment Act (ARRA) “Buy American” provisions. Location can also be important if you want to reduce the embedded energy from shipping.

However, PV module price drop has forced many U.S.-based module manufacturers to close their doors. Auxin Solar, Itek Energy, Mage Solar, SolarWorld, Stion Solar, Suniva, SunPower, and Silicon Energy offer modules produced in the United States (or at least assembled here, with components manufactured elsewhere).

Features

Integrated module features are primarily aimed at decreasing system installation time and materials needed. For example, modules with grooved frames allow rail-free mounting—interlocking straps secure the modules together and leveling brackets/feet secure the array to the roof. Grooved module frames compatible with railless mounting systems are available from Andalay Solar, Silicon Energy, and systems installed by SolarCity (a large national solar integration company that recently acquired Zep Solar). Additionally, Spice Solar has recently launched its built-in racking system for use with Auxin Solar “Spice-certified” modules. And for standard module frames, there are railless mounting options from Dynoraxx, PMC Industries, Roof Tech, S-5!, and Zilla.

Integrated grounding is a common additional benefit with rail-free mounting. Since the interlocking module-to-module components can also bond the module frames together, there is often no need for additional grounding on each module—only one array-to-equipment grounding conductor (EGC) connector is used to bond the entire array to an outgoing EGC.

Integrated Electronics

Some PV manufacturers (such as Trina Solar and UpSolar) offer integrated DC optimizers to provide module-level monitoring and maximum power point tracking (MPPT). This integrated feature reduces the installation time required to mount and wire DC optimizers separately for each module. Additionally, the DC optimizers on these “smart modules” are field-replaceable, so you don’t have to send the entire module back to the manufacturer to repair an electronics failure.

There’s no magic decision-tree for module selection, since the best module for your situation depends on the particularities of your site—and your personal design goals. But here are some “solar stereotypes” for a starting place toward choosing the right module.

Solar Stereotypes: Prioritizing PV Criteria

“Green, But Broke”

Some folks are on a tight budget, so module price trumps all other criteria—but this strategy also means DIY installation. The least-expensive modules come by the pallet, but you’ll need to factor in shipping costs. Getting a few friends together for a bulk purchase (and shared shipping expense) can be a good approach. On the flipside, if you aren’t up to DIY installation, it may be difficult to find an established installer willing to install products that are not purchased through their company.

If you’re “green, but broke,” consider financing or leasing a PV system, where little or no money down is required and the system can be paid off over time (years or decades). Or consider a power purchase agreement (PPA), in which you buy energy (kWh) from an onsite PV system installed and owned by a third party. It is important to note that when the system is owned by a third party, they are the ones who receive any available tax credits and other solar incentives. (For more information, see bit.ly/SEIAfinancing.)

“Hands Tied by an HOA”

Some homeowners associations (HOAs) regulate or won’t allow PV installations—usually due to perceived aesthetics. They may require the array be parallel-mounted flush to the roof (no angles opposing the roofline). Or the HOA may require that frames and back-sheets be dark-colored to better blend with roofs.

Many states (currently 41) have solar rights/access laws intended to ensure citizens can install solar systems on their properties. These laws vary widely; it is important to research and know your rights. For example, Colorado has a solar access law that doesn’t allow covenants to prohibit or place unreasonable restrictions on solar systems. However, the law does allow “reasonable restrictions,” so long as these limitations do not significantly reduce the PV system’s efficiency or increase system cost. (For more info, see bit.ly/SolarAccessLaws and dsireusa.org).

“Small Rooftop—or Bad Rooftop”

Many homes have limited available roof space for PV arrays, making module efficiency the top priority. And some regions require setbacks around the array for firefighter access, further limiting the array mounting area. If you are trying to reach a certain output size, your only option may be to choose high-efficiency modules—which come at a premium (about 20% more per watt).

Another factor may be partial shade from obstructions such as nearby trees, vents, or chimneys. When some modules will be shaded some of the time, using integrated microinverters or DC optimizers is a good choice to avoid compromising the performance of rest of the array, while avoiding increasing installation time. This approach also includes the benefit of module-level monitoring, which is helpful in assessing module performance and problems should they arise in the future.

If using high-efficiency modules or module-level electronics still doesn’t get you to your production goals, you may need to consider ground- or pole-mounts, or a PV awning or shade structure. At this point, module price will probably rise to the top of your criteria, since these mounting options are more expensive, requiring excavation, concrete, and a more expensive rack. Besides producing clean energy, a PV awning or shade structure can provide a covered outdoor space. In this case, you might want to purchase modules with a clear back-sheet, allowing some dappled light to shine through.

“Super-Green or Less-is-More Reductionist”

If you’re “super-green” or an “LIMR,” you’ll be interested in the extremes of reducing your system’s embodied energy and installation time. You’ll likely focus on modules with specialty integrated features, such as those that can accommodate railless mounting and integrated grounding. You will also prioritize module efficiency, since higher efficiency can also translate into less rack and wire per watt. Module manufacturing location will also be high on your list of criteria—and you’ll want to find made-in-the-USA products.

“Off-Gridder”

Battery-based PV systems have additional components that add to cost, so selecting lower-cost modules can help stay within the owner’s budget. Off-grid systems not only require batteries, but also battery boxes, large battery cables, charge controllers, DC and AC disconnects and overcurrent protection, and more expensive inverters. Also figure in shipping of these components (batteries are heavy) and more required installation time.

“Professional Installer”

When your job is to design, install, and maintain PV systems for the homes in your area, module reliability is crucial. You’re on the front line for the PV industry and your reputation is at stake. Since you’re responsible for determining which modules to use, you’ll be the one who has to deal with underperforming or failed modules. The movement toward comparative accelerated testing programs and quality assurance in module manufacturing is much-appreciated industry progress. You’ll likely be shopping for modules from reputable manufacturers, and with a sturdy warranty.

While it’s fun to present these stereotypes, most real-world installations will be a combination of the above scenarios. For example, I just purchased modules for my next PV array, and found that several of the categories applied—a local HOA to appease, a small rooftop, super-green. But more accurately, I am a solar nerd—and that means I want it all.

Web Extras

“Module-Level Performance” by Dan Lepinski • homepower.com/162.66

“Solar Equipment Innovations” by Rebekah Hren • homepower.com/157.38

“Choosing PV Modules” by Justine Sanchez • homepower.com/152.40

“PV Array Siting & Mounting Considerations” by Justine Sanchez • homepower.com/155.66

“Pump Up the Power: Getting More From Your Grid-Tied PV System” by Jeremy Taylor • homepower.com/127.72

“PV Array Output at Various Tilts & Orientations” by Justine Sanchez • homepower.com/155.32