Solar panels are a great investment that can provide you with cheap, renewable solar energy for the next 25 years or more. But a key question is, how do you maintain them? Are solar panels like your car, requiring regular tune-ups to stay in peak condition? Or are you supposed to just leave them be? Read on for the answers to the most common solar panel maintenance questions. ### Solar panel maintenance at a glance: - Solar panels require very little maintenance, especially if they are on a tilt, as rain can wash away any debris. - It is generally recommended that you perform solar panel cleaning between two and four times per year. Take extreme care when cleaning. - Clean from the ground, as climbing around on your roof can be dangerous. Use a soft-bristled cleaning brush and a non-abrasive cleaner. Be sure to clean the panels early in the morning before they get too hot from the sun; cold water and hot panels do not mix! - Solar panel maintenance and cleaning is not covered under panel warranties, but if your solar panels are damaged, they will usually be covered. ## Do solar panels need to be maintained? Solar panels generally require very little maintenance in order to function, so yes, you can usually just let them be. The only thing they need is a periodic light cleaning to make sure dirt, leaves, and other debris aren't obstructing the sun's rays. The only time you may need more extensive maintenance is during periods of heavy snowfall, as inches of snow can hinder solar panels performance. ## Can you monitor the health of your solar panels? If your panels’ energy output starts to decrease, you can monitor it via an app. Lower energy output could be a sign that your panels are obstructed by something, like debris, and cannot generate their typical electricity amount. By integrating a solar panel monitoring system into your setup, you can track how much energy your panels produce on any given day, and how much they produce over time. This allows you to see exactly how various external factors affect your panels' performance and how well they're maintaining their efficiency across their life. The monitoring system will also alert you if a panel breaks or malfunctions so that you can repair or replace it. Let's take a closer look at how to make sure your solar panels last as long as possible. ## How do you maintain solar panels? If your panels are tilted, you’re in luck: rainfall will actually clear away debris that's accumulated on them. However, during the dry season or extended periods without rain, it's important to clean them manually. **It is generally recommended that you perform solar panel cleaning between two and four times per year**. While that might seem like a lot, the good news is that this doesn’t require much work. All you really need is a leaf blower or a quick spray with a garden hose, and your panels will be clean and in top condition. In winter, you may need to clean your panels after a heavy snowfall. If you're using water to clear the snow away, make sure it's lukewarm. You can also use a squeegee with a long handle. It is important to note that you should never use hot water when clearing snow off the panels. The panels are made of tempered glass, and the extreme temperature disparity between hot water and cold panels can crack them. ### Ground-mounted panels vs. rooftop panels - is there a maintenance difference? When it comes to maintenance, the only real difference between ground-mounted solar systems and rooftop solar systems is that ground panels are easier to access, and therefore easier to keep clean. If you notice a buildup of snow or other debris, you can simply sweep it off with a soft-bristled broom. ## How does extreme weather impact your solar panels? Solar panels are designed to be durable. Top-quality solar panel installations allow PV systems to withstand all kinds of weather, from wind and rain to snow and more. You’ll be glad to know that solar panels hold up well against hail, as found by the Department of Energy. They can even survive hurricanes, as evidenced by the limited damage to solar systems in North Carolina during Hurricane Florence in 2018. Of course, they are not 100% indestructible; on rare occasions they can suffer damage by hail, hurricanes, tornadoes, or lightning. But as long as you have a good warranty, you should be able to have any damaged panels repaired or replaced. Somewhat ironically, the type of weather that most commonly affects panels is heat. When the temperature gets over 77°F (or 25°C), panels lose efficiency approximately 1% per degree. However, by elevating them a few inches above the ground or roof, you allow for air circulation, which can help cool them down and maintain energy production. Solar panel performance in hot temperatures is not something to stress too much about, your solar installer will take local climate factors into consideration when installing panels. ## How to repair or replace solar panels If your panels need to be repaired or replaced, it's best to hire a professional. If they're losing efficiency and still under a performance or power output warranty, then call the company that installed your panels. They will send someone to assess the situation and either fix the problem or replace the panels. If your panels have sustained physical damage, that's often covered under a separate equipment warranty. You can work with your solar installer to find out who can best replace those panels. Either way, don't attempt to install solar panels or make repairs yourself unless you're experienced with home solar systems. ### What is the standard warranty for solar panels? It is important to ensure that the solar panels you install come with a good warranty; if your solar panels' efficiency _does_ start to decrease, it can potentially cost you thousands of dollars over the years. If your solar panel is covered under a warranty, the company who installed it will provide a replacement. A good solar company will give you a warranty of between 15 and 25 years, and guarantee that your panels' energy production remains at 80% or above for that entire time. ## So, are solar panels hard to maintain? All in all, maintaining your solar panels is pretty easy. If you notice dust, debris or snow piled up, a quick clean will ensure that your panels operate effectively. Additionally, be sure the panels you get come with a good warranty so if your panels do break, they get replaced. Do that, and your panels should continue to operate at peak efficiency, bringing you clean, renewable solar power for years to come. Learn more about installing solar panels in your home in our beginner's guide.
Were you ever wondered about the number of solar panels required to power a home? As solar energy becomes increasingly popular, more homeowners are considering switching to this sustainable, cost-effective solution. But before you take the plunge, it's essential to know just how many panels your home needs to harness the sun's power effectively. In this article, we'll explore the factors that affect how many solar panels you require for your house, provide a step-by-step guide to calculating your solar panel requirements, and offer recommendations based on system size. So, be ready to bask in the sunshine of knowledge as we help you uncover the perfect solar panel setup for your home! ## What Determines the Number of Solar Panels Needed for Your Home? To know the precise number of solar panels needed to power a home, a formula involving three crucial factors is used: (1) annual energy consumption, (2) solar panel wattage, and (3) production ratios. Let's break down these components: 1. **Annual Energy Consumption** The initial step is to calculate your yearly electricity consumption, which refers to the total amount of electricity your household uses in a year. This figure, measured in kilowatt-hours (kWh), encompasses all electricity sources in your home, such as air conditioning units, lighting, water heaters, and other electric appliances. Based on research, an average household consumes approximately 11,000 kWh of electricity annually. 2. **Solar Panel Wattage** Selecting the ideal solar panels may seem challenging as they appear similar. However, their wattage varies, so it's essential to know the wattage of the panels you plan to install. Panel wattage refers to the electricity generated by the panel. Solar panels typically have a power output ranging from 250-400W. Therefore, it's reasonable to assume that the average panel wattage is about 300W. 3. **Production Ratios** The solar panel system's production ratio refers to the proportion between the system's estimated energy output over a period of time (measured in kilowatt-hours, kWh) and the actual size of the system (measured in watts, W). Although it might appear reasonable to expect a 1:1 ratio, the variability in sunlight levels can influence this assumption. To gain a clearer and better understanding, consider the following example: Imagine a 15 kW solar panel system installed in a sunny location like Arizona. This system generates 24 kWh of electricity annually. To calculate the production ratio, we would divide the annual energy output by the system size: - Production Ratio = Annual Energy Output / System Size - In this case, the production ratio would be: - Production Ratio = 24 kWh / 15 kW = 1.6 So, the production ratio for this solar panel system in Arizona is 1.6. This indicates that the system is highly efficient and able to generate a huge amount of energy due to the abundant sunlight in the region. On the other hand, a similar system installed in a less sunny location like Seattle might have a lower production ratio, such as 1.1, due to the reduced sunlight and cloudier weather conditions. ## Step-by-Step Guide to Calculate How Many Solar Panels Your Home Follow these easy steps to determine the appropriate solar system size for your needs: 1. Calculate your yearly kWh usage or your home's annual electricity consumption. You can refer to a year's worth of monthly electric bills or use the average house consumption figure of 10,649 kWh/year (average annual electricity consumption in the U.S.) as a basis for your estimate. 2. Determine the wattage of your solar panel, referring to the electrical power it generates under optimal conditions. You can approximate this value to be 320W for a simpler calculation. 3. Estimate the production ratio, representing the electrical power generated by your panel based on average sunlight exposure. To compute this, divide your solar panel's wattage by its annual electricity output. Alternatively, you can use the U.S. average range of 1.3 to 1.6. 4. Input the data into the formula: ****Panel number = kWh usage / production ratio / wattage**** - As an example, let's assume your house has an estimated 14,000 kWh consumption. With a production ratio of 1.5 and 320-watt panels, you would require: 14,000 kWh / 1.5 / 320 = 29. Therefore, you need 29 panels in total. - Now, let's talk about as well how many square feet of solar panels are needed. What you have to do is first know that an average residential solar panel is approximately 17.5 square feet. Next, determine the number of panels necessary by following the steps previously mentioned and multiply that number by the size of an individual panel. - Hence, in our example, a 14,000 kWh consumption would necessitate 507.5 square feet of solar panels (or 29 panels x 17.5 square feet). ## What's the recommended Number of Solar Panels Required Based on System Size? A typical homeowner may require approximately 28 to 34 solar panels to completely compensate for their electrical consumption. According to EnergySage, the following table offers a rough idea of the potential number of panels needed, depending on your home system size. |****SYSTEM SIZE****| ****NUMBER OF PANELS NEEDED**** | ****ESTIMATED ANNUAL PRODUCTION**** |--|--|--| |4 kW | 10 | 6,400 kWh | |6 kW | 15 | 9,600 kWh | |8 kW | 20 | 12,800 kWh | |10 kW | 25 | 16,000 kWh | |12 kW | 30 | 19,200 kWh | |14 kW | 35 | 22,400 kWh | Looking for the greatest solar panel for your household? Look no further! The [Anker 625 Solar Panel](https://www.anker.com/products/a2431) is here to brighten your day and power your life. With high conversion efficiency, you can harness up to 23% of sunlight into solar power, even on those pesky cloudy days! Featuring our proprietary Suncast technology, this smart solar panel aligns perfectly with the sun for maximum charge. And guess what? It's made for PowerHouse! Compatible with PowerHouse 521, 535, and 757 (sold separately), the Anker 625 is your all-in-one power solution for households and outdoor activities. Charge two devices at once with USB-C and USB-A output ports, and enjoy the lightweight, scratch, and weather-resistant design that lasts as long as your adventures. ## FAQ Here are some commonly asked queries on how many solar panels to power house. ### Is Solar Panel Installation a Good Choice for Your Home? Yes, solar panel installation is a good choice for your home as it offers numerous benefits. Solar panels help reduce electricity bills, increase property value, and promote environmental sustainability by capturing clean and renewable energy from the sun. However, it's also worth considering factors such as the location of your home, roof type, and the amount of sunlight your property receives to ensure optimal performance and return on investment. ### How Does My Home Size Affect the Number of Solar Panels I Need? The size of your home directly impacts the number of solar panels needed, as a larger home typically requires more energy for heating, cooling, and powering appliances. To determine the appropriate number of solar panels, calculate your home's energy consumption, assess the available roof space, and consider factors such as panel efficiency and local sunlight hours. Note that a customized solar panel system will ensure optimal energy production for your home size.
About half of sub-Saharan Africa’s population today does not have access to electricity. Those who do have electricity pay on average nearly twice as much as consumers elsewhere in the world. Power shortages cost the continent about 2 to 4 percent of GDP a year. And the large electricity needs will only grow in the foreseeable future. Given that the population in sub-Saharan Africa is expected to grow from 1 billion in 2018 to more than 2 billion in 2050, the demand for electricity is projected to expand 3 percent a year. This takes into account a steady increase in access to electricity as well as greater energy efficiency. Meeting that demand with current energy sources would have severe consequences for health and the environment. The current energy mix in Africa is based mostly on burning coal, oil, and traditional biomass (wood, charcoal, dry dung fuel). This reflects the energy resources of the continent, but also the use of technologies of the past. While this energy mix is comparatively cheap, it is insufficient to meet current needs, and negative effects on the environment are left unaddressed. The continent’s sources of energy will need to change, especially if African governments aim to achieve a healthy environment for their citizens and meet the emission limits for greenhouse gases set out by the 2015 Paris Agreement. ## Getting the energy mix right Fortunately, thanks to notable technological advances, Africa does not have to rely on large amounts of fossil fuel, as advanced economies did when they were at Africa’s current stage of development. There is the option to design an energy mix, built largely on renewable sources, that supports both strong growth _and_ low emissions. Apart from ensuring an ecologically sustainable approach to development, investing in renewable energy will also generate new job opportunities (IMF 2019). [](https://www.imf.org/en/Publications/fandd/issues/2020/03/images/032020/schwerhoff-chart1-lg.jpg) The right energy mix will allow Africa to develop rapidly while respecting the emission levels required under the 2015 Paris Agreement, in which governments commit to limiting global warming to 2°C above preindustrial levels. Chart 1 shows one such projection, in which the energy mix relies on a variety of technologies. The chart, based on projections made in 2013, suggests using modern biomass, cultivating high-energy plants, and using crop residue to produce synthetic fuels, as well as carbon capture and storage (CCS), which involves storing carbon dioxide emissions underground. Other researchers have proposed different mixes, all making use of these technologies (Schwerhoff and Sy 2019). However, these technologies carry risks. Biomass production competes with food cultivation and nature conservation. CCS has not yet been tested at an industrial scale. Both technologies can face resistance from local populations. To avoid large-scale reliance on unsustainable technology, Africa will need to move toward an economically and environmentally sound energy mix. This will require addressing the financial challenges of installing renewable energy capacity while seizing opportunities provided by falling prices and technological progress. ## Falling cost [](https://www.imf.org/en/Publications/fandd/issues/2020/03/images/032020/schwerhoff-chart2-lg.jpg) Prices for renewable energy have fallen substantially in the past few years, especially for solar power, whose cost decreased 77 percent between 2010 and 2018 according to the International Renewable Energy Agency (see Chart 2). While biomass, geothermal energy, and hydropower cost the least, these sources have limited potential. As illustrated in Chart 1, both geothermal energy and hydropower can reach a value that is several times larger than today’s generation capacity. The energy need, however, far exceeds this capacity. Geothermal energy can be very efficient (as we have seen in Kenya) but is available only in certain locations. Hydropower requires a careful balancing of environmental, social, and economic objectives. It is impossible to exploit the entire technical potential of hydropower: it requires the inundation of large areas, which threatens local ecosystems and often involves relocation of the local population. Hydropower is currently being hampered by continuous drought in southern Africa, and related energy generation has been severely curtailed in Zambia and Zimbabwe because dam levels are dangerously low. Conversely, there are large hydropower projects coming onstream or in preparation in west Africa, the Democratic Republic of the Congo, and Ethiopia. More promising for large-scale expansion of renewable electricity generation are solar and wind power, whose prices are now in the same range as those of fossil fuels. In addition, conditions for solar energy are excellent in Africa, where sunshine is not only abundant but also much more reliable than elsewhere. And investment into renewables is in fact picking up in Africa. South Africa, Uganda, and Zambia have held renewable-energy auctions that achieved competitive prices and attracted private investors. South Africa already has several solar power plants with a capacity of more than 100 megawatts. The Lake Turkana Wind Power project in Kenya is another success story. Despite successful examples in many countries, solar and wind accounted for only 3 percent of the electricity generated in Africa in 2018 compared with 7 percent in other regions of the world. The supply of electricity in Africa is strongly dominated by fossil fuels and to a lesser extent by hydropower (79 percent and 16 percent, respectively). The problem with renewable energy has always been that its supply fluctuates, posing a challenge for reliance on renewables as a source of electric power. Technological advances in stabilizing the electricity supply now make it possible for renewable energy to constitute a large share of the energy supply. These advances include using hydropower as a buffer during periods of peak demand, pooling electricity production from different geographic regions through a well-connected electricity grid, adjusting electricity demand to supply, and storing energy with flow batteries and hydrogen electrolysis. Currently, the share of variable renewable energy in total energy production is so low that variability is not yet a major concern. As this share increases, these options can be rolled out at a reasonable pace. With these technological advances, updates of Chart 1 show that it is possible for Africa to rely 100 percent on renewable energy by 2050 without slowing development. ## Overcoming financial challenges Financing is now the biggest challenge, however. Fossil fuel plants are comparatively cheap to build but expensive to run, as they require continued purchases of fuel. In contrast, renewable sources are inexpensive to operate but have high installation costs, which must be financed up front. Providing a high-quality energy basis for African development thus requires a comprehensive approach to financing (Schwerhoff and Sy 2017). If Africa is to take a new, low-carbon approach to development, its countries must mobilize public, private, and multilateral and bilateral donor financing to raise the funds needed for renewable-energy projects. On the public side, African governments can generate significant revenue by reducing the inefficiency caused by fossil fuel subsidies, which benefit mainly coal and oil. These subsidies are estimated at 5.6 percent of sub-Saharan African GDP (Coady and others 2019). Progressively phasing out subsidies—while protecting the vulnerable—could raise financing for renewable-energy projects. Moreover, African governments can potentially mobilize more of their domestic resources to cover the initial capital costs of renewable energy. For example, with an average tax-to-GDP ratio of about 14 percent in 2017, sub-Saharan African countries have ample room to increase their tax revenues. Use of carbon taxation could boost tax revenue while reducing fossil fuel carbon dioxide emissions (IMF 2019). On the private sector side, African countries must make substantial efforts to attract private investment to the renewable sector. Surveys have identified governance-related risks—complex bureaucracy and changing regulation—as the greatest threat to private investment in renewable-energy projects in Africa. Attracting private financing will require improvements in governance to reduce political risk. Reforming the financial sector to boost the incipient green bond market and reducing financial risk by transferring part of it to public actors can also help attract private investment. At the international level, multilateral financial institutions play an important role in facilitating long-term financing to support investment in climate change mitigation. In addition to identifying alternative sources of funding, these institutions provide tailored advice on the effective deployment of climate financing. The 2015 Paris Agreement is based on advanced economies’ commitment to mobilizing the equivalent of 0.12 percent of the world’s GDP a year through 2025 to address the needs of developing economies. Honoring this financial commitment would smooth the way for the transition to a low-carbon-energy economy across Africa—thecontinent with the lowest contribution to global warming. Only about 4 percent of global-energy-related carbon dioxide emissions in 2018 originated there (IEA 2019), yet Africa is the region most affected by climate change. This twist of fate certainly justifies more international support for the continent. **GREGOR SCHWERHOFF** is an economist in the World Bank’s Macroeconomics, Trade and Investment Department. **MOUHAMADOU SY** is an economist in the IMF’s Fiscal Affairs Department. Opinions expressed in articles and other materials are those of the authors; they do not necessarily reflect IMF policy. ###### References: Coady, D., I. Parry, Nghia-N.-P., and B. Shang. 2019. “https://www.imf.org/en/Publications/WP/Issues/2019/05/02/Global-Fossil-Fuel-Subsidies-Remain-Large-An-Update-Based-on-Country-Level-Estimates-46509Global Fossil Fuel Subsidies Remain Large: An Update Based on Country-Level Estimates.” IMF Working Paper 19/89, International Monetary Fund, Washington, DC. International Energy Agency (IEA). 2019. _[Africa Energy Outlook](https://www.iea.org/reports/africa-energy-outlook-2019)_ 2019. Paris. International Monetary Fund (IMF). 2019. _[Fiscal Monitor: How to Mitigate Climate Change.](https://www.imf.org/en/Publications/FM/Issues/2019/10/16/Fiscal-Monitor-October-2019-How-to-Mitigate-Climate-Change-47027)_ Washington, DC, October. International Renewable Energy Agency (IRENA). 2019. [_Renewable Power Generation Costs in 2018._](https://www.irena.org/publications/2019/May/Renewable-power-generation-costs-in-2018) Abu Dhabi. Schwerhoff, G. and M. Sy. 2017. “Financing Renewable Energy in Africa—Key Challenge of the Sustainable Development Goals.” Renewable and Sustainable Energy Reviews 75 (August): 393–401. ———. 2019. “[Developing Africa’s Energy Mix.](https://www.sciencedirect.com/science/article/pii/S136403211630778X)” Climate Policy 19 (1): 108–24.
Millions of Americans are deciding to power their homes with solar energy—especially as costs have decreased—but an investment in solar generates more than just clean energy. It can support household savings, energy independence, economic opportunities, grid resilience and security, and a safer planet. The U.S. Department of Energy Solar Energy Technologies Office (SETO) launched the [Connect the Dots on Solar Energy campaign](https://www.energy.gov/eere/solar/connect-dots-solar-energy) to shed light on the many benefits of solar energy and to help make sense of the solar energy landscape while providing a variety of ways people can do their part in the clean energy transition. Solar technology has a major role to play in the equitable transition to a decarbonized electricity system by 2035. As the director of SETO, I get to see first-hand solar energy’s impact on the nation and how we can do even more. For example, the [Community Power AcceleratorTM](https://www.energy.gov/communitysolar/community-power-acceleratortm), which focuses on growing access to solar for those who have been left out of the solar industry to date, is investing [$10 million in a prize competition](https://www.energy.gov/communitysolar/community-power-accelerator-prize) to accelerate community solar in underrepresented communities. Innovative practices like solar and agriculture co-location, called agrivoltaics, offer farms and agricultural businesses the opportunity to power their daily operations with solar energy and diversify their revenue streams. And our SolSmart program is working with hundreds of communities that want to enable more people and businesses to go solar. We want to ensure that Americans know how our work translates into real-world benefits for individuals, communities, the nation, and the planet. That’s why, throughout 2023, SETO will break down the various benefits that solar energy generates and highlight how SETO’s work helps to expand those benefits. SETO will tell the story of how our work grows and enhances the many benefits solar energy generates for consumers, businesses, communities, and the country at large. The benefits that the campaign focuses on include: - Creating household savings - Generating broader economic opportunities - Maintaining power through - and recovering from - extreme weather and disruptions, such as cyber threats - Owning your energy production - Benefitting the environment The decision to go solar is not one to be made lightly. That is why we are connecting the dots on solar energy: in the hopes of providing a big picture perspective of solar energy investments and their enduring, long-term benefits. Please join us on this year-long journey to better understand the different ways to get involved with and reap the benefits of solar energy. Follow along with our Connect the Dots content on social media and join the conversation using the hashtag #ConnectSolar and [sign up for our newsletter](https://www.energy.gov/eere/solar/solar-newsletter) for updates throughout the year. Thinking about taking the plunge into solar energy? Check out our [Homeowner’s Guide to Going Solar](https://www.energy.gov/eere/solar/homeowners-guide-going-solar) to learn about how to get the process started.
_Solar radiation_, often called the solar resource or just sunlight, is a general term for the electromagnetic radiation emitted by the sun. Solar radiation can be captured and turned into useful forms of energy, such as heat and electricity, using a variety of technologies. However, the technical feasibility and economical operation of these technologies at a specific location depends on the available solar resource. #### BASIC PRINCIPLES Every location on Earth receives sunlight at least part of the year. The amount of solar radiation that reaches any one spot on the Earth's surface varies according to: - Geographic location - Time of day - Season - Local landscape - Local weather. Because the Earth is round, the sun strikes the surface at different angles, ranging from 0° (just above the horizon) to 90° (directly overhead). When the sun's rays are vertical, the Earth's surface gets all the energy possible. The more slanted the sun's rays are, the longer they travel through the atmosphere, becoming more scattered and diffuse. Because the Earth is round, the frigid polar regions never get a high sun, and because of the tilted axis of rotation, these areas receive no sun at all during part of the year. The Earth revolves around the sun in an elliptical orbit and is closer to the sun during part of the year. When the sun is nearer the Earth, the Earth's surface receives a little more solar energy. The Earth is nearer the sun when it is summer in the southern hemisphere and winter in the northern hemisphere. However, the presence of vast oceans moderates the hotter summers and colder winters one would expect to see in the southern hemisphere as a result of this difference. The 23.5° tilt in the Earth's axis of rotation is a more significant factor in determining the amount of sunlight striking the Earth at a particular location. Tilting results in longer days in the northern hemisphere from the spring (vernal) equinox to the fall (autumnal) equinox and longer days in the southern hemisphere during the other 6 months. Days and nights are both exactly 12 hours long on the equinoxes, which occur each year on or around March 23 and September 22. Countries such as the United States, which lie in the middle latitudes, receive more solar energy in the summer not only because days are longer, but also because the sun is nearly overhead. The sun's rays are far more slanted during the shorter days of the winter months. Cities such as Denver, Colorado, (near 40° latitude) receive nearly three times more solar energy in June than they do in December. The rotation of the Earth is also responsible for hourly variations in sunlight. In the early morning and late afternoon, the sun is low in the sky. Its rays travel further through the atmosphere than at noon, when the sun is at its highest point. On a clear day, the greatest amount of solar energy reaches a solar collector around solar noon. #### DIFFUSE AND DIRECT SOLAR RADIATION As sunlight passes through the atmosphere, some of it is absorbed, scattered, and reflected by: - Air molecules - Water vapor - Clouds - Dust - Pollutants - Forest fires - Volcanoes. This is called _diffuse solar radiation_. The solar radiation that reaches the Earth's surface without being diffused is called _direct beam solar radiation_. The sum of the diffuse and direct solar radiation is called _global solar radiation_. Atmospheric conditions can reduce direct beam radiation by 10% on clear, dry days and by 100% during thick, cloudy days. #### MEASUREMENT Scientists measure the amount of sunlight falling on specific locations at different times of the year. They then estimate the amount of sunlight falling on regions at the same latitude with similar climates. Measurements of solar energy are typically expressed as total radiation on a horizontal surface,or as total radiation on a surface tracking the sun. Radiation data for [solar electric (photovoltaic) systems](https://www.energy.gov/eere/solar/solar-photovoltaic-technology-basics "Read more about solar electric (photovoltaic) systems") are often represented as kilowatt-hours per square meter (kWh/m2). Direct estimates of solar energy may also be expressed as watts per square meter (W/m2). Radiation data for solar [water heating](https://www.energy.gov/energysaver/solar-water-heaters) and [space heating](https://www.energy.gov/energysaver/passive-solar-homes) systems are usually represented in British thermal units per square foot (Btu/ft2). #### DISTRIBUTION The solar resource across the United States is ample for photovoltaic (PV) systems because they use both direct and scattered sunlight. Other technologies may be more limited. However, the amount of power generated by any solar technology at a particular site depends on how much of the sun's energy reaches it. Thus, solar technologies function most efficiently in the southwestern United States, which receives the greatest amount of solar energy. #### SOLAR ENERGY RESOURCE MAPS [View solar resource maps](http://www.nrel.gov/gis/solar.html) for both PV and concentrating solar-thermal power.
