Efficiency is a good thing. It can make cars go farther on a gallon of gas, and light bulbs brighter at the same wattage. But when it comes to solar cells, what exactly do we mean by efficiency? And how important are efficiency ratings when choosing a solar electric system? The answer may surprise you.
Simply put, the “energy conversion efficiency” of a solar cell is a measure of input vs. output. If 100 percent of the sunlight energy striking a solar cell could be converted into electrical energy, the cell would be “100 percent efficient.” However, the laws of physics apply to everything. Some energy always escapes through heat and reflectance — and efficiency is taxed further when individual cells are connected to form a usable solar “module.”
Today’s best-selling modules typically range from 12 to 18 percent in efficiency. Tremendous R&D investments throughout the industry keep these efficiency ratings rising. Companies like Kyocera pursue efficiency improvement as a strategic priority, because in the long term, better efficiency will make solar energy feasible in more and more applications. For these reasons, as buyers seek a common metric for comparing solar modules, it is natural to ask: Should efficiency be that metric?
Efficiency and Cost
For years, energy conversion efficiency was the most important variable influencing the cost of solar power. This was especially true when the raw materials for producing solar cells, like highly purified silicon, were in short supply. Greater efficiency meant getting the same amount of power from less silicon. However, the industry has evolved rapidly in recent years. Silicon supplies have expanded dramatically and their cost has stabilized. Finally, mass production has created unprecedented economies of scale — influencing the price of solar power more than the cost of raw materials. In fact, the declining price of solar modules over the past five years has had almost nothing to do with improvements in energy conversion efficiency.
Where Solar Efficiency Matters Most
Improving energy conversion efficiency is a strategic priority for most solar module manufacturers because it holds the key to opening new applications for photovoltaic technology. Efficiency improvement helped bring solar electricity out of the lab and into the NASA space program in the 1960s. It helped create terrestrial applications for solar energy in the 1970s, and it also enabled residential and commercial solar power starting in the 1980s. The dream of solar energy costing less than conventional power will be achieved someday, in part, through better efficiency. But what about today?
Today, solar modules with higher efficiency ratings generally cost more. Because of this, buyers may equate efficiency with quality and reliability, although they are not necessarily related. They may also assume that installing higher-efficiency solar modules guarantees you’ll produce more power. But is this really the case? In a word, no.
For today’s consumer, module efficiency matters most where space is limited. Obviously, your rooftop system can’t be larger than your roof. If the available space isn’t enough to meet your power goal, then you may need higher-efficiency modules. But for the needs of most home and business applications, space is not the limiting factor. In some cases, in fact, a user could install a greater number of lower-efficiency modules and actually produce more energy for the same total investment. It pays to compare.
What Are You Actually Buying?
For the consumer, it’s crucial to understand what you’re actually buying when you install a solar electric generating system. You’re buying electricity, measured in kilowatt hours. It is not the efficiency rating that runs your appliances; they run on kilowatt hours. For this reason, if you’re considering solar power, estimating the system’s long-term, real-world output, in kilowatt hours, is much more useful than comparing efficiency ratings.
The Dashboard Analogy
One helpful analogy is the dashboard of your car. The speedometer measures speed at any given moment, the way “instantaneous output” may be measured in a solar installation. Like a car, which rarely (if ever) operates at top speed, your solar installation reaches “peak rated output” only on the best part of the best sunny day.
A car’s odometer measures miles traveled, the way a solar electric system measures “kilowatt hours produced.” This is the most important gauge of a solar installation’s usefulness. Kilowatt hours are the power you use. If you can accurately estimate the number of kilowatt hours a system will produce in a year, you can estimate when your investment will pay for itself.
A dashboard “miles-per-gallon” readout displays your car’s efficiency. In a car, this is a very useful rating, since it tells you how far you can go on a gallon of gas. But on your roof, efficiency does not predict how many kilowatt hours a solar module will generate. In fact, two 300-watt modules with different efficiency ratings should generate the same amount of power, given the same sunlight exposure. For today’s buyer, the main difference is size — the higher-efficiency module is slightly smaller than a lower efficiency module of the same wattage.
Getting What You’ve Paid For
In the end, the “return” you receive from investing in solar power depends on three variables: cost, annual power production, and longevity. Your installer can inform you of the first – the cost to install and maintain your system over its expected lifespan. Your installer can also estimate the system’s annual production of kilowatt hours, which can help you estimate how many years it will take to recoup your investment. But what about longevity? Can you trust a 25-year warranty from a manufacturer that has only been in business a few years? This may be the most important question.
Fortunately, you can review unbiased research analyzing the performance of various technologies and brands side by side, and form your own conclusions.
Desert Knowledge Australia Solar Centre
Some of the most advanced research in real-world solar performance is publicly available from the Desert Knowledge Australia Solar Centre (DKASC), an independent demonstration facility for commercialized solar technologies. This government-funded public showcase features a broad range of solar power technologies from manufacturers like BP Solar, First Solar, Kaneka, Kyocera, Sanyo, Shurjo, Sun Grid, Sunpower and Trina Solar. Observing different solar installations under the same real-world conditions allows meaningful comparisons of performance among various brands. You may be surprised by what the data show. Anyone can view live data feeds comparing the performance of various solar technologies at no charge. Visit www.dkasolarcentre.com.au for more information.
In summary, if you’re shopping for solar, don’t get distracted by peak ratings or efficiency percentages. These are calculated under laboratory conditions and may not predict your system’s real-world performance.
Look for a solar module that delivers the most kilowatt hours per installed kilowatt — with minimal degradation over time. Has the manufacturer been around long enough to see its earliest products go out of warranty? Are its oldest modules still delivering trouble-free performance? Will that manufacturer still be around in another 25 years? The answers to these questions will predict your satisfaction better than any individual performance rating.
The opinions expressed in this article are solely those of the author Cecilia Aguillon, Kyocera Solar, Inc. Cecilia Aguillon is director of market development and government policy for Kyocera Solar, Inc., recognized as a world-leading supplier of solar electric energy products since 1975.