PV Module Selection

Advanced

Inside this Article

Typical PV array
A typical PV array.
SunPower PV module
Currently, SunPower offers PV modules with the highest efficiencies (W/ft.2). But unless array space is tight, efficiency isn’t always the most important criterion.
Frameless Lumos Solar array
Frameless glass-on-glass PV modules, seen in this Lumos Solar array, can look almost seamless.
SolarWorld array
Black frames and black back-sheets, like on these SolarWorld modules, can provide a more uniform appearance, but absorb more heat, which causes a slight decrease in efficiency due to increased temperature.
Silicon Energy transluscent modules
Clear glass backing in these Silicon Energy modules allows light to pass between the cells, making them desirable for shade structures.
Online module monitoring
Buying modules from long-term manufacturers with a good track record can be a hedge against poor quality, but the only way to spot individual module performance issues is with module-level monitoring.
Itek U.S.-made module
Advantages to U.S.-made PV modules include less embodied energy for transport; supporting the national economy; and, sometimes, as in the case of these Washington-made Itek Energy modules, additional state-sponsored financial incentives.
Spice Solar railless mounting
Some PV modules offer a rail-free mounting option. These modules are compatible with Spice Solar’s built-in racking system.
UpSolar module with DC optimizer
UpSolar’s “smart module” integrated SolarEdge DC optimizer reduces installation time compared to using separate MPPT optimizer units.
TrinaSmart module
Trina Solar’s TrinaSmart modules offer Tigo Energy’s module-level MPPT optimization and individual module monitoring.
DIY solar installers
If you have electrical and carpentry skills, you can potentially save a lot of money by installing a PV system yourself.
dsireusa.org screenshot
Visit dsireusa.org to see which states and localities offer solar access protection.
Awning-mounted array
Small rooftops may require using high-efficiency modules to meet your energy production goals. Poor rooftop orientation or convoluted shapes may necessitate a ground-, pole-, or awning-mounted system.
Professional PV installer at work
Professional installers may put a product’s reliability at the top of their module selection criteria in an effort to reduce warranty callbacks.
Typical PV array
SunPower PV module
Frameless Lumos Solar array
SolarWorld array
Silicon Energy transluscent modules
Online module monitoring
Itek U.S.-made module
Spice Solar railless mounting
UpSolar module with DC optimizer
TrinaSmart module
DIY solar installers
dsireusa.org screenshot
Awning-mounted array
Professional PV installer at work

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.

Pages

Comments (6)

JayP's picture

Hi Justine. Thanks for the quick reply and the tips. I’ve oriented the array to 173° True (per PVWatts based on our location of 39°N, 106°W) with a 35° tilt angle. (I know that I could adjust the tilt seasonally for a little better performance but currently achieve enough production to thankfully recharge the batteries back to 100% each day by noon.) You were also right about shading. I’ve kept a few pines from casting shadows over the years but will keep an eye on them. It doesn’t seem to take much shade to start impacting performance. I’ll also take a look at Lena’s article. I actually remember reading it when that issue came out and will review it again. For curiosity, I might also !!carefully!! check the Isc of each module separately with my clamp meter next spring just to verify performance at the module level. Otherwise, unless you can think of anything else, it sounds like I’m doing the right things and those “amazing” :-) 1550 watts were quite possibly “edge of cloud effect” related. Those numbers certainly looked good, though!! Thanks for help and have a nice holiday on your side of the Divide. Jay

Justine Sanchez's picture

Hi Jay,
Good question! From the numbers you list I don't see an immediate problem, in fact I am surprised you saw 1550 watts in the beginning. That might have been due to an edge of cloud effect boosting irradiance on the array. For a quick power check, normally you can take your STC array value (1470 watts) and multiply it by about ~0.75 as a standard derate (instantaneous reading on the input side of an MPPT charge controller) PV system AND assuming the charge controller is letting all power through. So then you are around 1100 watts…which is about what you are seeing. Now those fudge factors are pretty generous and thus that doesn't mean that there isn't possibly something also adding to your losses…so make sure you check for shading due to vegetation (trees & shrubs do grow, and can create shade that wasn't there previously). Are the modules dirty or dusty? Is the angle at the pole set for the correct season (i.e. steeper now that it is winter? commonly your latitude plus 15 degrees)? There are several factors that can reduce your output….including module age…on average they lose about ~0.5%/year. Here is a great article that will walk you through a performance check:
http://www.homepower.com/articles/s...

Cheers,
Justine Sanchez
Home Power Magazine
PS that equation you list is a simple efficiency check for your charge controller and doesn't tell you if the array is underperforming.

JayP's picture

Hi. I'm running an off-grid system that consists of a 48V battery bank (8 - 370 Ah batteries in series) and a 1470 kW array. The array consists of 6 - 245 watt panels that are wired as two parallel strings consisting of 3 modules each wired in series. The panel specs are Voc: 37, Vmp: 29.60, Isc: 8.75, Imp: 8.27. So, my total Voc @ STC is 111V and my total Isc @ STC is 17.5 A. For the first two years, I noticed great output on my FM80 during full sun. I'd actually see values around 1550 kW during bulk charging or running the microwave, etc. This past year, however, I'm only seeing around 1150 - 1200 kW under the same conditions. It also seems like my system is slower to wake up in the morning and goes to sleep sooner than compared to the same time in previous years. So, I'm wondering if a panel is failing. I know (think) that I can check performance by checking the Isc and Voc of each module during full sun. However, is there a way to check overall performance without having to disconnect each module? (My pole mounted array is surrounded by snow now which make access difficult.) I'm told that I can use the numbers from my FM80 to check performance (or efficiency?) using this formula:
Out V x Out A / In V x In A
Does that formula provide valuable information or do you have any other thoughts for determining the health of the array? I appreciate it.

akinyedani's picture

I have a 250W module with Voc = 37.4V, Vmp = 30.7V. Is it a good idea to assume that this module can charge 24V battery (2 units of 12V batteries). Which one can someone use for designing the charging voltage, Voc or Vmp?

Justine Sanchez's picture

Great question! You will use Vmp when considering if you have enough voltage to charge your battery bank. However you need more background info...These 60-cell modules, like the one you have, are pretty much the norm these days, with the industry now being directed at the battery less grid-tied market. Older solar modules were built on 36 and 72-cell designs to accomplish battery charging even under hot conditions for 12 and 24V nominal banks with standard charge controllers. However when batteryless grid-tied systems became an option the module design was no longer chained to battery requirements…and thus module designs shifted to the 60 cell design which works well for that market.

Anyway…it is important to remember that voltage drops with increasing temperature and while we call it a "24V battery bank" it actually has to charge to a higher voltage (the battery manufacturer supplies that set point, but it is often around 28.8V for a bulk charge on a flooded battery). Thus a module that can only deliver 30V under ideal STC conditions will deliver less as the module temp rises under more normal conditions, and likely* not have enough voltage to charge your 24V battery bank.
(note: you might be able to do this with Midnite Solar's Kid charge controller, you'd have to check with their array sizing tool, & you will have to factor in how much you want to spend on the system total)

It is more common to use a single 60-cell (~30Vmp) module to charge a 12V battery, Zeke Yewdall discusses this option in his MPPT charge controller article:

http://www.homepower.com/articles/s...

Cheers,
Justine Sanchez
Home Power Magazine

akinyedani's picture

Hi Justine,

thank you very much for this important information. I am grateful. Please do you think it is a good idea to couple 2 units of the PV module (series-connection) to charge 3 units of 12V batteries (series-connection)? My own context is the off-grid system. I heard people say that solar photovoltaic system is not reliable ? especially in developing countries, because of their previous experience with system failure. I want to tell these people that solar works, and that is why i like interacting with experts and people with sound knowledge here.

Show or Hide All Comments

Advertisement

X
You may login with either your assigned username or your e-mail address.
The password field is case sensitive.
Loading