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Solar energy systems which produce electricity directly from sunlight have greatly advanced in recent years. Now many municipalities and states are ramping up installations of photovoltaic or PV solar systems to produce electricity. After decades of research and development, studies find well-built solar systems can be reliable, resilient in severe weather, and economical. However, in a rapidly growing and evolving industry with intense price competition, training and quality assurance measures remain important. This document is a brief review of recent practical information on PV solar, including performance, quality assurance, installation, asset protection, and maintenance for stakeholders including investors, consumers, engineers and installers.
1. Progress in Price, Efficiency, and Storage
2. Real-World Performance and Warranties
3. Investing in Certified Installers and Equipment
4. Cleaning, Care and Maintenance
5. Managing Partial Shading
6. Weighing Investments in Solar Energy
List of Acronyms
The price of solar panels dropped by about 90% in 15 years–from an inflation-adjusted average of about $3.46 per watt in 2003 to 35 cents per watt in 2017. Price reduction continues for panels, installation, and in particular, soft costs.
The efficiency of solar cells and panels in converting solar energy into electricity has also increased. Consumers can now readily purchase silicon solar panels with conversion efficiencies around 22 percent. Efficiencies as high as 46 percent have been demonstrated for the most advanced concentrator solar cells, and efficiencies continue to rise for numerous types of panels, as well.
The payback time for U.S. residential systems depends on many factors, but buyers typically find payback periods to be shorter than the warrantied lifespans of solar panels. PVWatts® is a useful free calculator that investors and consumers can utilize to estimate system production and energy costs before investing in PV. This is provided by NREL, the National Renewable Energy Laboratory which also provides the System Advisor Model (SAM) to estimate performance and financial metrics for residential and commercial renewable energy systems, including battery storage and power purchase agreements.
For remote locations, solar combined with energy storage often provides an upfront low-cost alternative, eliminating the high cost of extending distribution lines from the nearest grid power source. The cost of batteries, which can store solar-generated power, is also dropping rapidly, facilitating off-grid installations, reducing demand charges, and providing valuable grid backup. NREL offers a tool called REopt, Renewable Energy System Integration and Optimization, which can be utilized to optimize resilience or financial savings for PV solar plus battery systems. BLAST, the Battery Lifetime Analysis and Simulation Tool Suite is another set of tools to estimate the useful lifespan of batteries under specific conditions before they need rebuilding or recycling.
Today, many buildings and homes can produce as much electricity as they use, especially if energy-efficiency measures are in place. Electric vehicles can also be charged with solar energy. Researchers have also found that homes with solar often sell for a premium, increasing the probability of a return on investment for the seller.
Well-constructed solar panels have demonstrated real-world lifespans of 25 to 30 years or more and performance has been analyzed in various climates. Certified solar panels are tested and designed to resist damage from hail, and high winds. Buyers in regions prone to tornadoes or hurricanes can use hurricane-resistant mounting brackets and consider bolts through solar panel frames. Selecting panels with thicker frames and thick, tempered glass also can increase resistance to damage from extreme weather and heavy snow loads. Well-built solar systems with energy storage can provide valuable back-up power when the grid is down. Warranties should be read carefully because they seldom cover all types of problems. However, a combination of warranties, certifications, and insurance can protect investments during extreme weather events and reduce financial risk.
Studies of newer panels are finding that failure rates are low, and systems often produce as much or more electricity than predicted thus far. To improve reliability of solar panels and systems, NREL continually analyzes performance and develops new international standards and recommended best-practices in collaboration with other experts globally. Standards and certifications may have helped the industry maintain reliability despite dramatic drops in price.
Solar panels often exhibit a gradual decrease in electricity production over time. This degradation rate is typically low—about ½ to 1 percent per year, and it is a relative number, starting from 100 percent of the panel’s original production level. For instance, if a panel that has an initial efficiency of 22 percent consistently degrades 0.6 percent per year, it would be 18.9 percent efficient 25 years later. Although warranties vary, manufacturers commonly guarantee power production at different solar panel ages in a linear or stepped pattern, which often amounts to about 80 percent of the original rated power output after 25 years. For some panels or systems, the warranty is backed by insurance in case the manufacturer goes out of business during the lengthy warranty time frame. Note, however, that many solar panels also have a more limited 10–12-year product warranty that covers issues such as failure due to manufacturing defects. A free, open source software, RdTools, provides a more reliable way to calculate degradation rates for systems that frequently log performance data.
Finally, inverters, which convert the system’s electricity from direct current to more commonly needed alternating current, are more likely to fail than solar panels. Therefore high-quality inverters that have longer warranties and quality certifications are recommended. Installing inverters in a shaded or sheltered area may improve lifespan by reducing temperature fluctuations.
Installers with high quality PV-specific training are recommended, such as those certified by the North American Board of Certified Energy Practitioners (NABCEP). Solar panels certified to stringent standards such as those of the International Electrotechnical Commission (IEC) are also recommended. The IEC standards identify whether a solar panel’s design is likely to exhibit known, early failures. Comprehensive quality assurance certifications including consistent manufacturing are also available through the IEC System for Certification to Standards Relating to Equipment for Use in Renewable Energy Applications (IECRE).
Utilize the roofing industry’s best-practice recommendations specific to roof type, such as flashing for durable, waterproof roof mounting. Researchers recommend using high-quality panels and installations that allow airflow beneath the panels, rather than mounting them flush to the roof, especially in warmer climates. Care should be taken to follow instructions specified by manufacturers because various panels and inverters require different installation and care. Panels with thinner frames and glass may require careful handling and mounting to avoid twisting or bending during transport and installation which could cause microcracks in cells. A technical guide for system installation is also available from NREL.
In practice, cleaning requirements for smaller systems are often minimal. Panels installed at an angle from horizontal of 5-10 degrees or more are often cleaned adequately by rain. Cleaning may be recommended if there is a lengthy absence of rain, a heavy coating of dust from construction, plowing, or a sandstorm, if debris or heavy, uneven soiling is visible on the panel surface, or if panels are mounted horizontally. In colder climates, steeper mounting angles also enable snow to slide off more rapidly.
If water is used to clean panels, researchers recommend hosing them down when they are cool so as not to thermally shock the panels. Most residential solar panels are made with crystalline silicon. However, using a squeegee to clean a type of solar panel with ‘thin film’ semiconductor material while the system is operating could cause damage due to partial shading. Shut the system off first. To avoid mechanical damage and performance issues, researchers also recommend not kneeling or walking on panels, unless the manufacturer notes that their modules have been designed for walking on. Good system design can reduce maintenance and repair requirements, and further useful information is provided in Best Practices for Photovoltaic Operations and Maintenance.
Solar panels may be partially shaded by a chimney or tree during part of the day, which negatively impacts performance. Fortunately, this issue can be reduced by selecting panels that tolerate partial shading and good system design. For example, commonly available system devices—power optimizers or microinverters—can enable electricity to flow from the panels that are in sunlight, instead of electricity flow being blocked by shaded or underperforming panels. In addition to reducing the impact of partial shading, power optimizers or microinverters also can enable the rapid shutdown of solar systems, a common safety requirement.
Distributed energy production combined with storage or special grid design can provide increased energy security during grid outages due to extreme weather or security problems. PV solar also has low water requirements, making it useful for regions with a limited or unreliable supply. Land use requirements for solar energy can be further minimized with distributed rooftop solar and other multi-use sites including shade structures (Figure 1). Quality solar panels also can pay back the amount of energy required for their production decades before retirement, and greatly reduce emissions in addition to financial savings. Compared to energy generation from traditional fuels, solar energy reduces pollutants that currently impact human health from infancy through adulthood, including cardiovascular and lung risks. Ratings for sustainability of manufacturing processes are also available, such as the Sustainability Leadership Objectives. Financially, region-specific incentives, as described in the DSIRE database, make investing in solar energy more affordable. Today, solar panels are often used to reduce energy expenses, provide grid back-up power, and power homes, vehicles, and commercial buildings. Solar energy systems have improved in efficiency, reliability, and price. Continuing to utilize and develop best practices and certifications drawn from more than a half century of PV experience and experiments can increase the return on investment and reduce risk for stakeholders.
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Battery Lifetime Analysis and Simulation Tool Suite
Database of State Incentives for Renewables & Efficiency®
International Electrotechnical Commission
IEC System for Certification to Standards Relating to Equipment for Use in Renewable Energy Applications
North American Board of Certified Energy Practitioners®
National Renewable Energy Laboratory
Calculator for estimating the energy production and cost of energy of grid-connected photovoltaic energy systems
Renewable Energy System Integration and Optimization
Degradation Rate Calculation Tool
System Advisor Model
Special thanks to David Feldman, Nate Blair, Garvin Heath, Whitney Painter, George Kelly and many others for contributing their expertise.
Sarah Kurtz earned a Ph.D. in chemical physics from Harvard University and joined the Solar Energy Research Institute (now the National Renewable Energy Laboratory, NREL) where she worked for more than 30 years. At NREL, she served as Co-director of the National Center for Photovoltaics and group manager for the PV Module Reliability Test and Evaluation Group. She is now a professor with the University of California, Merced, while continuing work with NREL. She is known for her contributions to developing world record breaking high efficiency multijunction, GaInP/GaAs solar cells, supporting the concentrator photovoltaic (PV) industry, and, more recently, her international work with PV performance, quality and reliability. Dr. Kurtz was a co-recipient of the Dan David Prize in 2007, the Cherry Award in 2012, and the C3E Lifetime Achievement Award in 2016. > Contact Sarah
Katherine Jordan is a technical writer and consultant for renewable energy, bridging information between stakeholders and scientists. An advanced statistics advocate for improving performance in R&D, she has a master’s degree in the interdisciplinary field of behavioral neuroscience. Jordan has 20 years’ experience editing and consulting in semiconductors, materials science and field performance of photovoltaics. For over a decade, she has researched concerns about solar energy from consumers, investors, installers, and engineers and communicated with scientists and other experts. > Contact Katherine