What Makes Solar Energy Renewable?
Currently, photovoltaic solar panels are roughly 20-22% efficient at converting the sun's electromagnetic radiation (photons) into the electrons it sends to the grid.2 But since the sun sends enough energy every 90 minutes to meet the world's annual energy consumption, efficiency is irrelevant in determining how renewable solar energy is.3 What is relevant is a metric called energy payback time, the time required to generate as much energy as it took to produce, use, and dispose of an energy-generating system. The energy payback time for a rooftop solar system is one to four years, meaning a rooftop solar system with a 30-year lifespan is 87-97% renewable, according to the U.S. Department of Energy.4 This is comparable to the energy payback period of coal; coal is highly energy-dense, so releasing it produces vast amounts of energy, enough for its energy payback time to be one to two years.5 The key difference with solar energy, however, is that unlike the sun's energy, coal itself is not renewable.
Is Solar a Green and Clean Form of Energy?
Because they emit zero greenhouse gases, solar energy systems are “clean” in their production of electricity, but studying the entire life cycle of solar panels (from raw material extraction to panel disposal) shows them to be less clean. How “green” solar energy is involves looking into areas beyond greenhouse gas emissions to the larger environmental impact in areas such as air pollution, toxic waste, and other factors. No energy production is completely clean or green, but when comparing the life cycle impact of all sources of power, solar is among the cleanest and greenest.
According to life-cycle assessment research conducted by the U.S. Department of Energy's National Renewable Energy Laboratory, a solar power plant emits roughly 40 grams of carbon dioxide for each kilowatt-hour of energy produced. (A kilowatt-hour, or kWh, is the amount of energy produced or consumed.) By contrast, a coal plant produces roughly 1,000 grams of carbon dioxide per kWh.6 Most importantly, while 98% of coal's emissions came from hard-to-abate operational processes (such as transport and combustion), 60-70% of solar's emissions come in upstream processes like raw materials extraction and module manufacture, which are easier to mitigate.7 The same applies to wider environmental impacts, such as the use of hazardous materials and toxic chemicals in both the production and disposal of solar panels, which can be mitigated by recycling, waste minimization programs, and changes in the manufacturing process, such as the use of cleaner sources of energy used to produce the panels.
How Sustainable Is Solar Energy?
Measuring how sustainable solar energy is means using a life-cycle assessment for all of its environmental impacts. What is the effect of solar power plants on land-use patterns and habitat loss? How much fresh water is used in the production of solar panels? What is the source of the energy used to produce solar panels, and how much greenhouse gas do they emit? How are the raw materials extracted, and how renewable or recyclable are those materials? And perhaps most importantly, how do all those assessments compare to the alternatives? For example, it may be more sustainable to produce solar panels in an area of the world with low levels of solar insolation (like high-latitude countries) and install them in areas where lots of the sun's energy reaches Earth (like low-latitude deserts), unless each of those areas contain fragile ecosystems or the transportation of materials halfway around the world involves burning more fossil fuels than the panels replace.
It's worth remember that all energy on Earth comes (or has come) from the sun. Ideally, the most sustainable use of that energy is the one that is most efficient at converting the sun's energy to usable “final” energy (whether for heat, transportation, manufacturing, or electricity) with the least environmental impact. While fossil fuels are energy-dense, they contain less than 1% of the solar energy that plants converted using photosynthesis during the Carboniferous period.9 This makes them far and away the least efficient and source of energy, independent from their environmental impact. Every energy source has many variables that need to be balanced in order to most closely reach that ideal, but none other than Thomas Edison, inventor, efficiency expert, and developer of the modern electricity grid, knew where to place his bets: "I’d put my money on the sun and solar energy. What a source of power! I hope we don’t have to wait until oil and coal run out before we tackle that."