PV Purchasing: Page 2 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

Cell Type & Efficiency. If you have limited mounting space, PV module efficiency is a key consideration. Modules with crystalline silicon PV cells—as opposed to thin-film—will likely be required, since thin-film modules produce about half of the wattage per square foot.

Within the crystalline module category, there are variances in power density (watts per square foot). Average power density is about 12.7 W per square foot, but some modules have higher power densities: Sanyo modules range from 14 W to 16 W per square foot; SunPower modules range from about 16 W to 18 W per square foot.

Using a module with higher power density means getting more power out of your usable mounting area. For example, let’s say our shade-free mounting area measures 20 feet by 10 feet, for an area of 200 square feet. Without considering module dimensions, choosing a module with a 12 W per square foot power density will yield a 2,400 W array; selecting a module that yields 18 W per square foot results in a 3,600 W array—a 50% improvement.

The downside is that modules with higher power densities typically are more expensive per watt (about 7% to 12% more). Decreased installation and racking costs for higher-efficiency modules may—or may not—amount to much. In most cases, if you have plenty of installation space, you won’t likely want the more expensive modules.

Bifacial modules that produce power from both the front and back of the module (Sanyo’s HIT 190 W and 195 W) report two values for module efficiency. These modules have clear backing, allowing some light to pass through, and can generate some energy from the reflective light that hits the back of the module. These modules can be good choices for awning and carport installations that can take advantage of reflected light.

Module Size & Dimensions. This will determine array layout—how many modules can fit in the available space. Differing roof planes, such as trapezoids (created by hip roofs) require carefully choosing modules with appropriate dimensions, so that roof space is maximized without hanging modules off the roof’s edges. Required setbacks for local fire department guidelines, accessibility for maintenance, module mounting infrastructure, and module interconnections and string layout will also influence what size modules will work in an array at your site.

Power Tolerance. This is the variance from the module’s rated output. For example, a 200 W module with a +/-5% power tolerance may produce anywhere between 190 and 210 W. Choosing modules with a positive-only power tolerance means the modules will at least perform to their rated specifications under STC, and possibly above. A 200 W module with a +10/-0 power tolerance is warranted to produce between 200 and 220 W. Because of higher output, they also may be appropriate for systems with limited installation space.

Voltage Characteristics. This will vary between modules. Maximum power voltage (module voltage under load and at STC) ranges from 16.5 to 72.3 volts; and open-circuit voltage (module voltage when there is no electrical load and it is exposed to sunlight) ranges from 21.8 volts to 88.1 volts. These values are for STC module cell temperature. Below  25°C (77°F), module voltage increases. Above this temperature, module voltage decreases. This, coupled with the need for the array to stay within the grid-tied inverter’s input voltage range, means that modules need to be selected and configured carefully. 

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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