Choosing PV Modules

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Inside this Article

Solar panels on a rooftop
Solar panels on a rooftop.
Garage roof is good location for solar panels
This large, south-facing garage roof has access to a wide-open solar window—the perfect location for a future PV array.
Module from Helios Solar Works
This 420 W module from Helios Solar Works is manufactured in Milwaukee, Wisconsin, and has a +3/0 power tolerance.
SunPower module
This SunPower 435 W module is 20.1% efficient. High-efficiency modules allow a higher-capacity array to fit within a limited area.
Peak Energy module from REC
This Peak Energy module from REC has a PTC-to-STC ratio of 0.91.
SunPower Signature black solar modules
Using high-efficiency modules can reduce the mounting space required. This 12 kW array uses SunPower Signature black solar modules for aesthetics.
Silicon Energy Installation
In some programs, purchasing components made in-state qualifies the system for larger incentives.
Lumos Solar frameless modules
An awning that uses Lumos Solar frameless modules with clear back sheets allows light to filter through.
Trina Solar modules
Zep Solar-compatible modules from Trina Solar have grooved frames to accommodate hardware for rail-less mounting.
Upsolar modules
Zep Solar-compatible modules from Upsolar have grooved frames to accommodate hardware for rail-less mounting.
Jinko Solar module
This Jinko Solar 245 W module has 10-year materials and 25-year linear power warranties.
PV modules can be both a science and an art
Every residential installation is unique—and selecting PV modules can be both a science and an art, where production, budget, and aesthetic goals are balanced within a given space.
Solar panels on a rooftop
Garage roof is good location for solar panels
Module from Helios Solar Works
SunPower module
Peak Energy module from REC
SunPower Signature black solar modules
Silicon Energy Installation
Lumos Solar frameless modules
Trina Solar modules
Upsolar modules
Jinko Solar module
PV modules can be both a science and an art

Once again, life has shifted for my family of four and we are looking for our next home. While the home’s square footage, layout, and yard space are considerations, the feature I tend to get most excited about is where we can mount our next PV array. We are considering a home that has a large garage roof. Its excellent southern solar exposure and the lack of pesky vents and dormers to work around has got me thinking again about PV array design—and module selection. There’s ample mounting space, which could accommodate increasing the array size to not only meet 100% of our electricity needs annually, but also support a future electric vehicle.

There are a lot of modules to choose from (see the “Online PV Module Guide” sidebar) but there is also uncertainty about the survival of many PV module manufacturers because of the large supply-versus-demand imbalance that currently exists, along with the challenge for many manufacturers to produce their products at today’s cheap module pricing. These factors—combined with the usual considerations, such as module output ratings, power tolerance, efficiency, and pricing, along with new inverter, mounting, and module-level electronics options—make module selection more complex. To help me wind my way through the choices, I’ve devised a list of top considerations that will be helpful for any array design exercise. Additionally, a module selection example is provided, given our potential roof space and energy generation goals.

Power tolerance is a measurement of how close a module’s actual output will be to its rated output under standard test conditions (STC: cell temperature = 77°F and irradiance = 1,000 watts/m2). For example, if a 200-watt module has a power tolerance of +/-3%, its actual output (under STC conditions) can vary from 194 W to 206 W. Some modules have a positive-only (such as “+5/-0”) power tolerance, which means that these modules should be able to produce at least rated power under STC, and possibly more.

PTC ratings (PTC-to-STC ratio) specify module power output for settings that more closely represent real-world conditions, which makes them lower than STC ratings. The STC temperature of 77˚F for a module’s cells is often not a very realistic temperature for these dark cells exposed to direct sunlight; their temperature will commonly be much higher. As cell temperature increases, voltage drops, which reduces module power output. PVUSA test conditions (PTC) calculate module output using an ambient air temperature of 68˚F (at 1,000 watt/m2 irradiance), which typically causes cell temperatures to be about 113°F to 122°F (36°F to 45˚F higher than STC).

However, modules are sold based on their STC-rated power output rather than by PTC ratings, making it more difficult to compare realistic performance between modules. A PTC-to-STC ratio is included in the table for all modules. The closer the PTC rating is to STC, the higher the module output is under more common conditions. For example, if a “200-watt” module has a PTC-to-STC ratio of 0.9 or better, then its PTC rating should be 180 W or higher; if the ratio is 0.85, then its PTC rating will be only 170 W. Although that difference may seem negligible, when you add the power up for an entire array, it can be significant.

Module voltage and string inverter input window need to be considered for any grid-tied PV project that uses a string inverter. Each module has a specific maximum power point and open-circuit voltage, and each site has specific temperature ranges it will experience, which will determine the actual voltage each module will operate at.

Additionally, each inverter has its own input voltage limitations, which will dictate string size for module models being considered. Many string inverter manufacturers have online sizing calculators to help find string configurations that work for each PV module, considering local climate.

Comments (4)

Justine Sanchez's picture

Hi Stephanie,
Thanks for posting your comment about Colored Solar modules. Folks can check out their specs and compare to other modules on the list if you go to: http://www.homepower.com/web-xtras
and click on "2012-2013 PV Modules Buyer's Guide"
Best,
Justine
Home Power Magazine

stephanieb's picture

These factors are all important when comparing apples to apples, but introduce an aesthetic US made pv panel like from Colored Solar into the equation. 1) Do we need to put a imported product on every roof in America? 2) Does solar have to detract from traditional American architecture? 3) Homeowners are proud of their home's look and appearance and have to only choose black to outfit their home with 4) Colored modules perform better, than black panels

Michael Jacobs's picture

I really enjoyed the article. I shared it with my students. We were discussing the selection of pv cells for student design projects. The article explained the criteria used for making their pv cell selection which was helpful to the class in their student design project. Worked very well with a real world experience. Love this magazine. I have learned so very much from the articles. People know me by the magazine i carry with me at all times. Thanks so much!!!

Michael Welch's picture

Hi Michael. Thanks for the kind words, and we're glad you and your students are finding good value in the magazine.

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