Solar Efficiency Losses Over Time

by Shawn Roe | 3 Comments

How much do solar panels decrease over time? The solar industry standard is a conservative estimate of 3% in the first year, and less than 1% per year after that. However, solar panel manufacturers are starting to realize that this is too conservative, and they are beginning to warranty their panels to more realistic degradation rates.

There are three points I want to make.

  1. Use the solar panel’s warranty to compare output loss over time because that is the only output you are guaranteed.
  2. Realistically expect less than a 3% decrease in output the first year, and about .5% decrease per year after that for most panels.
  3. One solar panel from 1979 was tested in 2010 (after 30 years) and its output was better than the original factory specs.


First, the industry standard for solar output warranties is 90% output in year 10, and 80% output in year 25. This is the 2011 warranty for companies such as JA Solar, First Solar, Yingli Solar, Canadian Solar, Sanyo, and Sharp. Right now there is not enough data to know reliable degradation rates after 20-25 years, so you must rely on manufacturer warranties in order to compare expected solar output losses over time.

Now, most solar panels won’t degrade this much. This is just want the manufacturers will guarantee. So, some manufacturers are starting to update their warranties to set themselves apart from the competition. Here is a list of the best warranties available from major solar panel manufacturers.

  1. SolarWorld and Trina Solar both offer linear performance guarantees which basically means that they are guaranteeing only the small decrease for each year up to 25 years. Technically, it protects you a little more. Check both of their websites to see how they explain it.
  2. Suntech protects you at year 5, 12, 18, and 25, which gives you more protection than the standard.
  3. Sunpower warranties 90% to year 12, which is 2 years longer than the standard.
  4. Every other solar company is the standard – 90% at year 10, and 80% at 25.

Here is a chart comparing the solar output warranties of 10 major companies. Click to enlarge.

Solar Panel Output Loss Comparison

Lastly, solar panels will last for more than 30 years. They were originally designed to withstand the conditions in space and to be used in remote places without electricity access. There are no moving parts in a solar panel. The cells are encased in metal and glass frames designed to withstand hurricane winds, rain, hail and other extreme weather conditions.

Martin Holladay bought a panel back in 1980 that was manufactured in 1979. In 2010, he tested the output of his solar panel for the 30 year anniversary. It performed better than expected, returning 97% of the original voltage and more amps than the spec sheet said it should output originally. You can read more here.

Solar will last a long time. Expect a 2% decrease when you install them, and then a reasonable 0.5% per year after that. Let’s hope this article helps improve the 2011 industry standard warranties.

Solar Manufacturer Warranty Documents

Notes: 1. Trina Solar has two different warranties for Mono and Poly modules.
2. Manufacturers were chosen based on 2010 market share and name recognition.


3 comments on “Solar Efficiency Losses Over Time

  1. I live in Arizona where there is tons of dust and very little rain, all year. I appreciated seeing this article on efficiency loss of solar panels. It has become clear to many people that having solar panels cleaned regularly helps solar panels operate at their peak performance.
    From Scientific American magazine & Ceramic Tech Today: “The effects of the dust on these solar energy system are tangible. “A dust layer of one-seventh of an ounce per square yard decreases solar power conversion by 40 percent,” explains MIT visiting professor Malay K. Mazumder. “In Arizona, dust is deposited each month at about four times that amount.”)

    And this article says: “In the desert you can lose up to 80 percent of performance,” said Professor Heinrich Häberlin of the PV Laboratory of the University of Engineering and Architecture in Bern, Switzerland.”
    (http://www.pv-magazine.com/archive/articles/beitrag/let-the-light-shine-through-_100005421/86/?tx_ttnews%5BbackCat%5D=192&cHash=4caddfb91d234ed7cfb8c52fa24062ef#ixzz29O8kIz33http://www.pv-magazine.com/archive/articles/beitrag/let-the-light-shine-through-_100005421/86/?tx_ttnews%5BbackCat%5D=192&cHash=4caddfb91d234ed7cfb8c52fa24062ef#axzz29O63BNtm)

    While I would never argue that cleaning them will slow or stop degradation of the panels over time, I do wonder why so many solar companies still tell their customers that they don’t need to clean them at all. Any ideas or statistics you could share on that subject?

    Full disclosure: I own a window cleaning company that cleans solar panels.

  2. An installer has come to sell me on solar and gave me a quote to install some Trina panels. I asked about cleaning them, and they said it wasn’t necessary: periodic rain/snow will clean them. Nevertheless, some amount of dust will be present most of the time. Unfortunately, I will have no ability to clean these panels, as the sun-facing portion of my roof is 3 stories high (walk-out basement + 2 story house). Factoring the purchase and installation prices, the margins are very tight, and it’s a long time before I’ll see a return on my investment. How closely to factory efficiency would my panels produce, assuming they’re only cleaned by wind/rain/snow? (I understand that production is variable to the amount and direction of sunlight, but taking that all out and just thinking about the dust that may collect, is it fair to assume a certain % reduction in generation – would it be fractions of a percent, or potentially multiple percentage points?)

    1. Well, the common AC-DC derate factor from soiling (dust and dirt) is 5%. Your installer should’ve given you an ‘expected first-year production’ number, and would’ve included this (and other) degradation factors into the calculation. The average tends to be around 23%. Hopefully your ROI calculation was based on the expected production, instead of on the sticker-rating of the panels.

      If you have more questions, feel free to contact me directly and I’d be glad to try and clarify anything for you.

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