Solar Panels Get Less Efficient Over Time. Don't Worry About It

Residential solar installations have seen a spike in recent years, with many Americans considering transitioning their energy usage to renewable sources (especially in light of new federal tax credits).

If you're among those on the fence about solar, you might be wondering how long your solar investment will last -- and how efficient your solar panels will be in the next 20 years. The good news is your panels are likely to work just as well in the future. 

While the efficiency of solar panels does drop over time, it's usually not a big enough change to be a major worry, according to Joshua M. Pearce, a materials engineer who researches solar power systems at Western University in London, Ontario. 

"When a module or a PV system fails, it would usually be from something catastrophic, such as a tree fell onto your house and busted up a bunch of your panels," Pearce told CNET.

Here's what you need to know about how your solar panels' efficiency changes over time.

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What is solar panel efficiency?

Today's solar panels have efficiency ratings in the upper teens to lower 20s. That means when photons from the sun hit the solar panels on your roof, about a fifth of those photons are absorbed and converted into electricity. The photons that aren't converted to electricity either bounce off the panels (like a reflection) or are absorbed, but not converted into electricity. This is because many of the sun's rays, like those in the infrared spectrum, can't be absorbed by today's solar panels. 

While the efficiency of today's solar panels may not sound impressive at face value, Pearce said the technology is actually astounding compared to the efficiency of natural systems.

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"The most efficient biological conversion of sunlight into anything is under 2%," Pearce said. "We are 10 times better than the fastest-growing plant that has existed on Earth before humanity got here. We're doing pretty well."

Ultimately, the efficiency of solar panels should not be a major concern for consumers. When a solar installer gives you an estimate for your house, they will compensate for the efficiency of each panel by calculating the number of panels necessary for your home's power needs.

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"Every single system is designed for a particular house," said Freddy Petkus, founder and owner of Critical Mass Solar, a Massachusetts-based solar installer. "We do enough panels to offset your energy use no matter what kind of efficiency the panel has."

How does solar panel efficiency change over time?

Solar panel technology has come a long way over the past few decades, but we're far from creating a perfect solar cell. Given these inefficiencies, solar panel manufacturers expect a degradation rate of about 0.5% a year, Pearce said, and their warranties will cover any panels that fail to meet those expectations. However, this is rare.

"When you look at the data, most modules actually degrade even less than that, maybe 0.1%, and they last much longer than 25 years," Pearce said. 

How to track solar panel efficiency

If you want to keep track of how much electricity your solar system is producing, there's an app for that. These apps, which most solar companies provide, also allow you to track how much energy you're consuming. While this can be valuable information for homeowners with solar systems, you'll have an extremely difficult time tracking the efficiency of your panels with these apps.

The variation in how much solar energy your panels get from day to day and year to year will drown out any visible effects of degradation in panel efficiency, Pearce said. "The average consumer has no chance of finding a 0.1% drop in efficiency with their system."

If you want to invest in equipment to track efficiency, Pearce recommends the installation of DC optimizers or micro inverters on your solar system.

Will solar panel efficiency improve in time with new technology? 

Technology inevitably gets more efficient and powerful over time. According to Pearce, solar panels won't necessarily improve due to technological breakthroughs, but rather because of better manufacturing techniques. These advancements will lead to solar panels with better glass that can absorb more solar energy, thinner layers of metal to allow for more cells and better positioning of the metal contacts.

More efficient cells will also get darker and blacker, Pearce said, "more like Darth Vader's helmet, where nothing is coming off of it, you just see the evil on the inside as they suck all the light in, then turn it into electricity."

You should only wait for these improvements before investing in a solar system if you want to burn your money, the experts CNET spoke to said. 

"Right now in Massachusetts, the breakeven point with the major utility companies is anywhere between six to nine years to pay for solar," Petkus said. "Hypothetically, if your bill is $200 a month, in 10 years you'll have paid about $24,000 to the utility company." This is money better spent upgrading to solar. 

Treat switching to solar power much like you would any other investment, Pearce said. Analyze the rate structure of your power provider to calculate the rate of return on installing a solar system. Then, try to factor in future rate increases and inflation. An estimate of about $3 per watt to install a solar system is a good baseline. The math should speak for itself.

"Solar cell efficiencies will definitely improve, but waiting around for them won't get you anywhere," Pearce said. "This is a 25-year guaranteed rate of return with no taxes, because it's all savings, and it's inflation-proof."

More on solar panels

• When Do Solar Panels Pay for Themselves?
  
• Why the Type of Solar Panel You Get Matters
  
• How Big Are Solar Panels? Here's Their Typical Size and Weight
• What It Takes to Go Off the Grid With Solar Panels