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Solar Panel Ratings Breakdown

Solar panels, to this point, are fairly complex structures. Not everyone can buy, slice, and incorporate an ingot of silicon into a solar panel. While the actual electricity-producing function of a solar cell is not too difficult to comprehend, the manufacturing process is incredibly complex and therefore expensive, as anyone who has priced out a solar system would agree.

solar-roof.jpg
Photo Credit: Abi Skipp

With such esoteric information to digest, it would be easy for manufacturers and dealers to scam customers into buying cut-rate products. That is why solar panel ratings (not to mention warranties) exist. There are several ratings that should be specified for every solar panel and each is important in its own right. Just because ratings exist does not mean we cannot be scammed. So understanding and inquiring about solar panel ratings is of the utmost importance and will ensure that you get the most out of your solar system.

Minimum Warranted Power

This is a manufacturer rating. Essentially, the manufacturers will guarantee that their panel’s actual power rating will not be less than a specified amount coming out of the box. This may have several names including peak tolerance rating, minimum warranted power, and negative tolerance rating. Regardless of name given, this means that a 200-Watt solar panel, with a negative tolerance rating of 10% (or a minimum power rating of 90%) is only warranted for 180 Watts out of the box.

Remember to look for the highest minimum warranted power rating and/or the lowest negative tolerance rating. Both are saying the same thing with different numbers.

STC vs. PTC Rating

California and many other states require solar panel manufacturers to submit operating specifications for their products. These products are then further tested by an independent laboratory. At least in California, the agency approved to do this is PVUsa. The results of PVUsa tests are dubbed a panel’s PTC rating (PVUsa Test Conditions).

Therefore every panel will have both an STC rating (standard test conditions) and PTC rating. The PTC rating involves more extreme, real world test conditions, so you’ll find it to be lower than the STC rating and more indicative of how a solar panel will hold up. So the higher PTC rating, the better.

Also, it is the PTC rating that the state uses to calculate system rebates (i.e., expected performance). So while the STC rating for a panel may be 175 Watts, if the PTC rating is only 160 Watts, then that will be the benchmark for your cash rebates — an important tip when figuring expenses and payback period for your solar system.

Efficiency Ratings

This is the rating we’re all familiar with and the one that gets so much attention these days as solar panel developers and researchers struggle create the high efficiency solar panel. Efficiency rating is vital because the higher the efficiency, the more power you’ll get per square inch of panel surface. In other words, the higher the rating the less area you’ll have to use to achieve the same amount of power production. Be sure to ask your dealer for a factory printed spec sheet that lists solar module efficiency (not simply solar cell efficiency) and the other ratings listed above and below.

solar-ratings.jpg
Photo Credit: functoruser

UL Listing

Underwriters Laboratories (UL) is a product rating company that has been testing products for safety for over a century. In terms of solar panels they test for mounting method, weather resistance, performance, and other safety considerations. In 2008, UL opened a massive PV testing site in Silicon Valley. You will commonly hear products that pass UL’s rigorous testing as “UL listed.”

Recently one of our readers inspired this post by asking why Uni-Solar’s SHR-17 solar shingle had lost its UL listing. That, as it turns out, is a very good question. All that is apparent is that Uni-Solar lost its UL listing for that product and discontinued manufacturing the solar shingle altogether. I spoke to a UL representative about it he could not disclose information on an “inactive” file. Whether or not Uni-Solar discontinued manufacturing before or after it lost its UL listing is unclear.

What we do know is that UL still rates products for safety, including thin-film solar panels, and that Uni-Solar no longer makes solar shingles. They instead focus solely on “thin-film solar laminates that convert sunlight to energy using proprietary technology.”

Regardless of Uni-Solar’s particular situation, a UL listing is a positive sign and just about every major brand of solar panel will have it as safety is of high concern in system design.

SRCC Ratings

Those interested in a solar water heater or swimming pool heater should pay attention now. In order to qualify for federal tax incentives for a solar hot water system, that system must meet standards set by the Solar Rating and Certification Corporation (SRCC).

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Posted on April 28th in Solar Information by Dan.

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8 Responses to “Solar Panel Ratings Breakdown”

  1. Steve Says:
    May 14th, 2009 at 4:49 pm

    So what are the top three performing solar panels for residential use on the market?

  2. ECD Fan Says:
    May 29th, 2009 at 6:15 am

    To Steve: The “top performing” solar panels may not be a good choice, as they could be really overpriced or may not fit your roof well. For example, NASA uses multijunction GaAs solar panels in space, but they are a bit costly… Here is the top-12 list of panel manufacturers, ranked by their KW sales (non-cancelled reservations) over the past 17 months in California for “residential” systems (systems rated 10KW STC or less), according to the California Solar Initiative:

    Sunpower Corporation 15891
    Sharp 9964
    BP Solar 6519
    Kyocera 5766
    Evergreen Solar 5510
    Sanyo 4361
    Mitsubishi 3449
    Andalay Solar, Inc. 3157
    SolarWorld California, Inc. 1837
    First Solar 1673
    Suntech Power, Inc. 1365
    REC ScanModule AB 991

    More on how calculations were performed, here: http://ecdfan.blogspot.com/2009/05/csi-snapshot-as-of-april-29th-2009.html

    Note to CalFinder: Power/efficiency degradation is quite important, as well. Certain data from NREL, for example, indicate that Unisolar’s shingles (those same SHR-17) and the rest of the triple junction a-Si product line degrade over 1% a year (average linear degradation). That will mean the product’s power output will be less than 80% that of its original rating by the time the warranty expires (Unisolar has a short, 20-yr warranty).

  3. Stephanie Says:
    October 14th, 2009 at 10:12 am

    Besides UL, are there are other third-party rating agencies for PV panels or specific “grades” that UL applies to each panel they test?

  4. ECD Fan Says:
    October 20th, 2009 at 5:24 am

    The IEC certification (EN is the corresponding European equivalent) is the gold standard in PV certifications (for example, UL 1703 is expected to be replaced by IEC/UL 61730 within two years):

    The details here:

    http://www.pvresources.com/en/standards.php

    However, a certification by itself is not guarantee. For example, the Unisolar PV laminates ignited on the rooftop of the Long Beach Convention Center in February of 2008, shutting down that 750KW installation. Yet, they still obtained their IEC certification in July of 2009:

    http://ecdfan.blogspot.com/2009/06/certification-of-safety.html

  5. CW Says:
    November 22nd, 2009 at 9:36 am

    I’ve noticed that not all specification sheets state the Efficiency Rating of the module and sometimes just show the rating for the cell. From what I’ve seen, all cell efficiency rating are higher than the module rating. Also, is there any other reason for a person to be concerned about efficiency rating other than it effects the size of the module in relation to power output? Would a person need to be concerned about efficiency if there was sufficient area available to place the modules?

  6. Dan Says:
    November 23rd, 2009 at 4:23 pm

    Cell efficiency is higher than module efficiency simply because anytime power is transferred over distance (even through a solar module) some of that power will be lost in the process. While a solar cell may convert 16% of solar energy into electricity, only, say, 14% may actually leave the module/panel as usable electricity.

    Not only does efficiency affect module size, it can also affect the size and cost of the entire system (i.e. less efficiency equals more panels necessary and subsequently higher cost). However, if you have sufficient space to provide all the power you need or desire, than it is not too big of a deal.

    You have hit upon the double issue facing solar cell/module researchers and developers: how to increase module efficiency while simultaneously reducing costs and spatial requirements.

  7. Alternative Energy Guide Reviews | PV Solar – Homeowner Analysis (3) Says:
    November 28th, 2009 at 9:19 pm

    [...] “Solar Panel Ratings Breakdown” Tags: alternative energy [...]

  8. Bob Butler Says:
    March 1st, 2010 at 11:00 am

    Greetings-Im looking for the California Energy website that shows the ratings of all the pv panels. Can you help me? Thanks!

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