Author: Lacy Cook — You probably have a vague idea of how solar operates: panels on your roof, taking in rays of sunlight, and turning them into electricity. But how exactly does the magic happen? If you’ve ever wondered just how solar power works, this solar 101 blog post is for you.
How does solar energy work?
In about one hour, enough sunlight reaches Earth’s atmosphere to provide electricity for every human for a year, according to National Geographic. And we’ll keep receiving this massive supply of potential power for the next five billion years. It only makes sense to harness this incredible source of renewable energy — and currently, we accomplish this one of two ways: photovoltaic cells and solar thermal technologies. Residential rooftop solar utilizes photovoltaic cells.
What is a photovoltaic cell?
A photovoltaic cell is the same thing as a solar cell. There are typically 60 or 72 solar cells in a solar panel. Inside each solar cell is a semiconductor, usually silicon. When light reaches a semiconductor, that material absorbs energy from the light. The energy dislodges electrons, ultimately creating an electrical current in the solar cell.
But homes can’t use that electrical current quite yet. It’s direct current (DC), or electrical charge flowing in one direction. Homes use alternating current (AC), or electrical charge that regularly reverses. An inverter makes the change from DC to AC so you can power your home with the sun.
Solar cells are protected by glass, and the module is held sturdy by an aluminum frame. One reason why solar cells are so long-lasting is because the only moving parts within them are electrons.
Fun fact about silicon: much like the sun, it’s a very abundant resource. Maybe not five-billion-years abundant, but it is the second most plentiful element on Earth, after oxygen. It can be sourced from sand, and then refined to create the crystalline silicon found in solar cells.
What do I need for a rooftop solar array?
Solar panels don’t just sit directly on your rooftop. You need what’s called racking equipment, to attach the panels to your roof and keep your system in place when it’s windy or stormy. Second, you need equipment to transform DC into AC, like an inverter, wires, and breakers.
You may also be wondering how many solar panels you need, and the answer depends on how much available roof space you have, and how much electricity you consume.
What side of my roof is best for solar and why?
A south-facing roof is the best side for solar. From our perspective in Washington DC, because of the tilt of the earth and since we’re north of the equator, the sun will appear to be moving through the southern part of the sky, rather than directly overhead. South-facing roofs will tend to receive the most light. However, east-west roofs still receive plenty of light too. It’s only north-facing rooftops that may not make as much sense for solar.
However, it is important to consider trees and shading. If you have a south-facing roof that’s shaded much of the day by tall trees, solar may not make as much sense on that side of the roof. Some shading can be okay, but if one side of your roof is completely shaded, it may not be beneficial to install solar there.
Roof angle matters too, but not as much. The most ideal angle for year-round production is a location’s latitude, which is almost 39° here in DC. On a roof, a 39° angle is pretty steep! For context, common roof angles are between 18° and 35°, and flat roofs tend to be between 0° and 10°. Luckily, roofs that aren’t exactly 39° (few are!) can still generate plenty of power. According to EnergySage, panels on a roof with a typical pitch facing south will lose just a few percent of production compared against a roof with an ideal tilt.
Do I need batteries for a solar panel system?
You do not need batteries for a solar system. However, there are a few reasons why you might want battery storage. If you have one, it’ll hold excess electricity your panels generate, and you can power your home at night, on rainy days, or during a power outage with that electricity. You’ll avoid paying your utility company their fluctuating rates, and you’ll be able to power more of your home with renewable energy. Although you’ll still remain connected to the grid, you’ll vastly reduce your dependence on it.
Batteries aren’t cheap though, and if none of these reasons are a draw for you, or if you don’t consume a lot of electricity, you might choose to forgo installing a battery at this point. We can always add one to your system later if you change your mind.
I heard solar panels are only around 20% efficient; why aren’t they 100% efficient?
We still lose a good amount of the sunlight that reaches a solar cell. Temperature, reflection, the angle of the panel, and differing wavelengths mean our solar cells aren’t yet 100% efficient — in fact, we’re not even close to 100%. The average efficiency, or the percentage of solar power hitting a solar cell that is turned into electricity, is indeed around 21% for residential rooftop solar panels, with the most efficient (and expensive) panels at 22.8% efficiency.
But we’re making progress. In 2020, scientists at the National Renewable Energy Laboratory created a solar cell with an efficiency of 47.1%. The materials they used are still expensive and not cost-effective for residential solar, although researchers are working to lower costs.
If you’d like to explore more of the science behind solar panel efficiency, the Office of Energy Efficiency & Renewable Energy offers a useful guide.
How long do solar panels last?
Solar panels last 25 to 30 years (although some estimates even say 40 years). After a few decades, they won’t stop producing power, they’ll just do so with less efficiency. Since solar panels rely on the sun to operate, they’re necessarily exposed to snow, rain, wind, humidity, and other DMV weather, and their electricity output degrades over time — around 0.5% a year, according to the National Renewable Energy Laboratory. That means after 25 to 30 years, depending on your panels’ degradation rate, they’ll be producing around 80% to 90% as much power as they did the year they were installed.
How many rooftop solar arrays could power the planet?
Okay, this is more of a fun question, but turns out there’s a theory to answer it, thanks to a 2021 Nature Communications paper from researchers at University College Cork, Imperial College London, Ahmedabad University, and Columbia University. They estimated only 50% of rooftops around the world would need solar arrays to meet everyone’s yearly electricity demand. In the researchers’ words, their study “is the first to provide such a detailed map of global rooftop solar potential, assessing rooftop area and sunlight cover at scales all the way from cities to continents.” And they identified three hotspots to generate this solar energy: Europe, Asia, and North America. Just another reason to go solar in DC!
If you’re ready to add your roof to the clean energy movement, call us at (202) 750-5718 or schedule a consultation via the form directly below. We’d be delighted to answer any questions you have about solar.
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