The Facts of Light: What are the different types of solar energy technology?

By SolarCity

May 20, 2016

We get a lot of questions about solar power. That’s why we’ve introduced “The Facts of Light” ‑ a place where you can inquire about all things solar, and we’ll do our best to get you the answers.

Today, we’re diving into the different types solar technologies and which ones are right for your home.


The term “solar energy” simply refers to the energy that comes from the sun, generally in the form of light or heat. Much of the energy we use each day can be traced back to the sun’s energy. Think about it. Biofuels like ethanol and wood are the result of plants turning sunlight into energy to grow. Wind is a result of different levels of heating from the sun around the Earth, causing air to move that we then capture with wind turbines. And yes, even fossil fuels like oil and coal are the result of plants turning sunlight into stored energy millions and millions of years ago. These plants then died, got buried deep beneath the ground, and gradually morphed into fossil fuels.

So if most energy is from the sun, what do we mean then when we say “solar energy technology”? Well, solar energy technologies are those that turn sunlight directly into useful energy, like electricity. Solar energy options cut out the middle-man, or the middle millions of years, as the case may be.

There are two main technologies used to turn sunlight directly into electricity – solar panels (also known as solar photovoltaics, but we can just say “solar panels”) and concentrating solar power. Solar energy can also be used to provide heating for buildings directly, without any conversion to electricity. We’ll cover all of these.

Solar Photovoltaics


Solar PV is the most common (92%) of all installed solar power generation in the US and is what providers like SolarCity install on people’s rooftops (and in some cases on the ground).

Solar photovoltaics are what most people think of when someone says “solar power” or “solar panels.” They’re also the main technology we’ve installed for our 230,000+ customers. The word, “photovoltaics,” comes from the Greek word for light (“photo”) and “volt”, the unit that measures electric force. And that’s exactly what solar panels do: turn sunlight into electricity.

The scientific principle that makes solar panels work – the photovoltaic effect – was first discovered in 1839 by Alexandre-Edmond Becquerel, a French physicist. But it wasn’t until the 1940s that the first solar panel was actually made. Today, however, solar panels have become an integral part of the U.S. energy mix and are used on many different scales, from single-family homes to large, utility-owned installations. Residential solar was the fastest-growing segment of the solar industry in 2015, as the cost of installing and owning solar panels has fallen drastically and more and more people have realized the benefits of solar-powered homes.

In fact, there are enough suitable rooftops for solar panels in the US to satisfy nearly 40% of the country’s electricity demand. Because the fuel is free, all that clean energy has the potential to deliver a lot of money savings on Americans’ utility bills.

Concentrating Solar Power

The second main type of solar energy technology, concentrating solar power, is quite different from solar panels. The technology uses reflectors (think of them as mirrors) to concentrate sunlight onto a small, high-efficiency solar collector where it heats up a fluid such as water or molten salt or synthetic oil. That heated fluid produces steam, which is used to drive an electric generator.


Concentrating Solar Power uses mirrors to reflect and concentrate solar energy onto a collector (the long pipe in this photo), where it can heat up a fluid that drives a steam-powered electric generator. (Source: Department of Energy)

Concentrating solar power plants operate at a massive scale, with the smallest concentrating solar power plants generating roughly 1,000 times the power of the typical home rooftop system. But concentrating solar power plants take a long time to build, take up a lot more space than solar panels, and can cost hundreds of millions, if not billions, of dollars to build. It’d be pretty hard to fit one of these on your roof!

Solar Heating

Some solar technologies don’t convert sunlight into electricity, but instead use the sun to provide direct heating to buildings. Solar water heating, for example, can provide hot water for a building by running water past a solar energy collector (pictured below), where the sun’s energy heats water before sending it back to the building. Just heating water can account for 14% of a home’s energy use, so offsetting this cost can make a real dent in one’s utility bill.


Solar water heaters, like these, collect the sun’s energy and use it to heat water running through a nearby pipe. (Source: Pixabay)

Solar energy can also be used for temperature control in buildings without any special equipment. Have you ever curled up with your favorite book in a sunny spot in your house to stay warm? You’re using solar energy! The strategic use of sunlight can increase people’s comfort: architects and designers can design rooms with specific building materials and orientations to make the rooms feel comfortable all day long. When you combine this kind of design with electricity-producing solar panels, it’s actually possible to achieve a net zero carbon emissions building!

Solar power is easier to use today than ever

The innovation and improvements in solar energy technologies continue to be exciting and are probably well beyond what Edmond Becquerel ever thought possible when he proposed how to turn sunlight into electricity more than a century and a half ago. Even with all the improvements we’ve made over the years, much of the solar power used around the world still employs the same basic concept Becquerel discovered. We’ve simply made it cheaper and more efficient to install solar panels, which is why the use of solar power is spreading so fast. After all it only takes a day to install a system that can harness an energy source that will keep providing us energy for the next 5 billion years! Sounds like a bright idea.


Want to learn more on how tapping directly into the sun’s energy can cut your utility bill? Give us a shout here.

Stay current: the week’s best solar links

By SolarCity

May 13, 2016


We’ve rounded up our top reads of the week about solar and the future of the energy … so you can stay current.

Cleaner sources of electricity like solar panels are helping drive a promising trend in the fight against climate change: America’s energy-related CO2 emissions in 2015 fell to a three-year low. (Today in Energy / Energy Information Administration)

Despite winter darkness, solar power may have a brighter future in Alaska than one might expect: a recent report from the National Renewable Laboratory finds that solar “can be economically competitive in many remote Alaskan villages and could have a number of benefits, including reducing a village’s dependency on diesel fuel, improving electricity price predictability, providing local environmental benefits, and more.” (Annie Zak / Alaska Dispatch News)

Airplanes currently account for 3% of humanity’s carbon footprint and are also one of the fastest-growing sources of global warming pollution. Several interesting strategies to cut emissions are in the works – including biofuels, futuristic wing designs, and even fuel cells. (Brad Plumer / Vox)

The humongous wildfire continues to burn in Canada, destroying communities and thousands of acres of forests. Elizabeth Kolbert examines the link between the Fort McMurray fire and climate change. (Elizabeth Kolbert / The New Yorker)

Renewable-energy leader Germany just set a stunning new record. On the morning of May 8, renewables covered a record 95% of the country’s electric demand. Solar power’s share of the total peaked at 26.11 gigawatts -- the largest of any energy resource (Sandra Enkhart / PV Magazine).

The Facts of Light: How Long Do Solar Panels Last?

By SolarCity

May 12, 2016


We get a lot of questions about solar power. That’s why we’ve introduced “The Facts of Light”—a place where you can inquire about all things solar, and we’ll do our best to get you the answers.

We live in a throwaway society. Even our phones and our laptops -- incredibly sophisticated, high-tech and well-designed pieces of equipment -- only last a few years before we need to replace them. We might get our cars to last more than 10 years if we can put up with some repairs and diligent maintenance, but, frankly, most things aren’t very durable.

Solar panels, by comparison, are in it for the long haul. There are no digital processors, no delicate motherboards, no flywheels, no pumps and no fans. There are very few moving parts that can break or wear out, and as a result they require almost no maintenance. The components of the panel are durable and include a glass-protected solar cell (often made of silicon) that captures sunlight. A solar panel sits in one place and absorbs the sun. That’s its job.


So, even those old-school panels that went up on the homes of early adopters decades ago are still producing electricity pretty efficiently. SolarCity assures its customers that its solar panels will last approximately 30 years with little to no maintenance. The US Department of Energy’s National Renewable Energy Lab (NREL) has published some studies on the durability and effectiveness of solar panels over time, and they found the 30-year estimate is conservative. The truth is there are panels out there that are more than 30 years old and still going strong. NREL’s study found that as solar technology has improved, the long-term performance of panels has only gotten better.

Today’s solar panel systems are just as humble and hard working as the early ones, but they are also lighter-weight, can produce more energy, and are far more stylish.


Is there a catch?

Full disclosure: After 30 years go by, your solar panels will not be quite as efficient as the day you put them up on your roof. Annually, solar panels’ performance decreases by about half a percent. This small loss in efficiency is mainly due to exposure over time to water vapor and temperature fluctuations that wear on the coatings and materials. As a result, panels are only about 86% as efficient after 30 years. That’s nothing to sneeze at, of course. Can you think of any other product that works 86% as well after it’s been operating for 30 years?

In addition, solar panel systems have a special piece of equipment called an inverter, which may need to be replaced every decade or so. Inverters take the electricity produced by a solar panel and niftily convert it into power that’s ready to be used in your home. Inverters do have a few more parts and wiring involved, but they’re still highly durable, and new, high-quality ones are expected to last more than 14 years. No need to stress though: all of SolarCity’s systems come with a full 20-year protection plan, so if any part of your system needs repair or replacement, Solar City will fix it at no additional cost.

Lastly, because your solar panels may stay with your home longer than you do, your warranty and all the terms of your financing agreement transfer easily to the next person to inhabit your solar home if you decide to sell. 

In it for the long haul

Most durable things are resilient because they are simple and made from quality materials. Solar panels, made from crystalline silicon covered in very durable glass, are like that. There’s a reason they use them in space. For one thing, it’s one of the only feasible ways they can generate electricity in space, but also it’s because solar panels can endure in even the harshest conditions.

When your solar panels’ lives finally do come to an end, SolarCity will recycle them. All parts of the panel are separated out into materials streams and made into new products. In a world where nothing lasts, a solar homeowner can always glance up at the roof for a little reassurance that the disposable society hasn’t permeated every aspect of life.

Stay current: the week’s best solar links

By SolarCity

May 06, 2016


We’ve rounded up our top reads of the week about solar and the future of the energy … so you can stay current.

-The US has now surpassed 1 million solar installations. And the two-millionth installation is fast approaching, likely within just a couple of years. (Julia Pyper / Greentech Media)

-A key reason that solar has spread so fast is its tendency to become “contagious” within communities. In many American cities, a remarkably high proportion of solar customers have been referred by a friend. (Brad Plumer / Vox via SolarCity blog

-Some of climate change’s most dire consequences may come through disruption to water resources, according to a new report from the World Bank. The findings add urgency for shifting toward energy technologies that avoid carbon emissions and reduce water use. (Chris Mooney / Washington Post)

-A study across 11 solar farms in the UK concludes that solar panels and vibrant biodiversity can go hand in hand. The results reveal that the solar farms, in combination with thoughtful land management strategies, are correlated with a positive impact on plant and animal life, including broad-leaved plants, butterflies, bumblebees, and birds. (Sam Pothecary / PV Magazine)

-Late-night show host Jimmy Kimmel taps into humor to argue that climate change should be a matter of science, rather than political opinion. (Paige Lavender / Huffington Post)

The remarkable reason that solar is going viral in these 10 American cities

By Barry Fischer

May 04, 2016

Solar power continues to grow at extraordinary speed across America. More solar panels were installed in the U.S. in 2015 than in any year in history, and 2016 is expected to deliver twice as much. This is the year that America surpassed its millionth solar installation, providing an historic amount of clean energy.   


Source: GTM Research / Solar Energy Industries Assocation (2016) 

Why is solar growing so fast? A few key factors: the cost of solar panels has plummeted, the importance of solar is critical to combating climate change, and pro-solar policies have become mainstream.

But even these primary factors don’t address a more practical question. In particular, how exactly do American households decide to go solar?

In many cases, the answer can be traced to something most of us do every single day: talking to friends and neighbors.  

In fact, more than one in three SolarCity customers have gone solar because they were referred by a friend or neighbor – who was actively spreading the word about the benefits of rooftop solar power.


This vibrant network of solar enthusiasts is the basis of our “Solar Ambassador” program – which has been instrumental in driving SolarCity's more than 230,000 installations nationwide. Thanks to our Ambassadors, solar power has often become “contagious” across communities. Seeing solar panels on a neighbor's rooftop and word-of-mouth conversations can both be key motivators to go solar, as verified by a fascinating study from Yale University researchers.

Several US cities stand out as being remarkably “contagious” for the spread of solar – places where the majority of SolarCity customers have originated via word-of-mouth referrals. We studied our residential installations through the end of 2015, and zeroed in on the localities where solar is going viral thanks to our Ambassador referral network. Here’s what we found.

The Top 10 Most Contagious Solar Cities in the U.S.

Our top 10 most “contagious” solar cities are dotted across several states, from Hawaii to New Jersey. They span distinct geographies, climate zones, and demographic profiles. But they all have in common a boom in solar panel installations thanks to word-of-mouth referrals.

Taking the top spot for solar contagiousness is Fort Collins, Colorado (69% of its solar customers were referred by a friend), closely followed by Kona, Hawaii (64%) and Gloucester Township, New Jersey (62%).


What is it that makes these cities highly “contagious” for rooftop solar? It certainly has something to do with people’s outspoken passion for solar energy there. But we can also speculate that it has something to do with sociology and community vibrancy – wherein neighbors and friends are enthusiastic about connecting, sharing information, and helping each other.

For example, Colorado’s prominence on our list of contagious solar cities may not be a coincidence, but rather stems from the state’s tight-knit social dynamics. Among SolarCity territories, Colorado has some of the highest rates of people who “talk to their neighbors” (88.2%) and are “active in their neighborhood” (10.8%), according to the US government’s Corporation for National and Community Service. And Fort Collins in particular has one of the highest rates of volunteerism (38.2%) in the country among mid-size cities – implying the kind of community cohesion and social capital that is conducive to encouraging each other to go solar.

Only six states are represented in the list above, but other states have standout cities, too. As shown in the map below, there are several other communities where a large proportion of solar customers were referred by a friend. Consider Harwich, MA and Brentwood, NY – both of which can thank peer referrals for catalyzing nearly half (47%) of all solar rooftop installations.


What does solar contagiousness look like? 

Solar installations have been cropping up in the US at an eye-popping rate in recent years. Residential solar’s growth rate has exceeded 50% for four years running. In the maps below, you can get a sense of this rapid growth, and the dramatic role that word-of-mouth referrals can play.

Every dot appearing in the maps represents a new SolarCity rooftop, and green dots specifically represent rooftops that came by way of a word-of mouth referral. The predominance of green dots in Fort Collins (our #1 most solar-contagious city) and nearby Greeley (#9) shows how fruitful word-of-mouth referrals have radiated across Northern Colorado.


We’d be remiss not to point out the small town of Windsor in the middle of the map, where a whopping 73% of SolarCity rooftops have come from referrals (hence the many green dots). Coincidentally, Windsor was also recently ranked as the best place to live in Colorado – perhaps because life is better when neighbors encourage each other to go solar! (The only reason it didn’t make our top-ten list was because it doesn’t quite have 100 solar installations in total, which was the threshold for inclusion in the list.)

Similar to Northern Colorado, the Honolulu Area (#6 in our list) further demonstrates how word-of-mouth referrals can fuel the spread of solar throughout a community. Interestingly, in the map below, you can see how some Hawaiian neighborhoods’ adoption of solar appears to be almost entirely referral-driven (nearly all green dots), like around ‘Ewa Gentry and Waipi‘o.


Solar Ambassadors and their word-of-mouth referrals have visibly helped push Hawaii to achieve America’s highest solar adoption rate (nearly one out of five of households in the state have gone solar). On the island of Oahu, some 32% of single-family households live underneath a solar rooftop.

Five thousand miles east of Honolulu, the influence of Solar Ambassadors is also evident in suburban New Jersey. In Gloucester Township (#3 on our list), more than three of five SolarCity rooftops can be traced to word-of-mouth referrals. New Jersey ranks fourth place nationally for total solar power capacity, in part thanks to this strong neighbor-to-neighbor dynamic.


So, who exactly are the Solar Ambassadors who are the driving force behind all those green dots we’ve seen above?

Who are Solar Ambassadors?

SolarCity’s community of Solar Ambassadors consists of more than 200,000 people nationwide who are passionate about promoting solar to others in their network – whether it be in the neighborhood, at the office, or on Facebook and Twitter.


Many Ambassadors are proud SolarCity customers themselves, who enthusiastically refer their friends and family to follow suit. Still other Ambassadors may not have the right roof for solar themselves, but proactively advance the solar movement by educating others about the benefits and conveying how SolarCity can help.


The fact that more than one in three SolarCity customers comes to us through a referral is worth celebrating for numerous reasons: it means there is a strong excitement about solar in American communities; it’s a cost-efficient way to find new SolarCity customers (>80,000 from referrals and counting); and it’s also a gift that keeps on giving. In particular, we find that customers who were referred by a friend are subsequently more likely to refer others – and to refer higher numbers of them.

Of course, all this solar contagiousness is good for the planet, but it’s also pretty sweet for the pocketbooks of Solar Ambassadors and the friends they refer. Each time a referral of an Ambassador installs a SolarCity system, the Ambassador gets a $200 hat-tipand the newly referred solar customer gets 1 month of solar electricity on us (in addition to securing an affordable, zero-carbon power supply for years to come).

Join your neighbors.  Join the solar movement.

Interested in becoming a solar ambassador yourself? Learn more here.


Special thanks to: Fiona Li, Matt Ewing, Priscilla Lopez, Yuri Yakubov, and Camila Perez-Goddard.

Methodology note: Data is based on SolarCity installations and referral activity through December 31, 2015. Only cities with greater than 100 SolarCity residential installations were considered for inclusion in our rankings of top cities.

The designation of “Eastern Orange County” encompasses the communities of Trabuco Canyon, Rancho Santa Margarita, and Coto de Caza; the designation of “Greater Honolulu” encompasses the City of Honolulu as well as the nearby communities of Waipahu, Kapolei, and ‘Ewa Beach.

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SolarCity's mission is to accelerate the mass adoption of clean energy. Follow solar’s progress here.

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