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Fight Junk Science - Solar Panel Efficiency


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Whenever something technical becomes popular, junk science appears. Solar energy is one of those subjects littered with junk science. Junk science is often behind the specifications given for solar energy systems, such as the key figure of efficiency. Sales people are very creative in how they specify the efficiency of the system they are trying to sell you. They may talk about the conversion efficiency of their solar cells in a photovoltaic (PV) system, or the efficiency of the power inverter that converts the DC output from solar cells or batteries to AC for use in the home, or the optical efficiency of the mirrors in a concentrating solar power system. Those are important numbers for the system designer, but not for the end user. So ignore all that and look at the basic measure of efficiency.

The basic measure of efficiency of any energy conversion system such as solar is simply the output power divided by the input power, a simple ratio that must always be less than 1. Simple. You just need to know those two numbers. Normally the output power is readily available. It's usually the first spec given by the data sheet for the system or by the salesman. But what about the input power? Although the marketing types might make it sound complicated and mysterious, determining the input power is very simple.

The input power is the amount of solar power striking the collection area. There are two numbers you need to know to figure that out, the size of the collection area, and the amount of solar power per unit area striking the earth's surface. The size of the collection area is easy to calculate. It's the size of the solar panel, that is, length times width (for a rectangular solar panel which is typical). The second number is even easier to obtain because it is a well established number for the amount solar power that strikes the surface of the earth. That number is 1,000 watts per square meter of direct normal solar insulation at sea level. The number varies somewhat depending on your altitude and weather conditions. And depending on the type of collector the solar energy system uses there may be some other contributors to the total energy collected. But this standard number is sufficiently accurate for use in evaluating solar energy systems.

So how might this knowledge be useful to you? Here is a real example. At the time of this writing there is a company demonstrating a solar panel with a claimed output power of 3,000 watts. The company claims they have breakthrough technology that makes it many times more efficient than existing solar panels available from other companies. They are seeking investors to help them build a factory and ramp up production. Sounds like a hot investment, doesn't it? Let's look at the basics of their product.

The solar panel being demonstrated measures 3 feet by 5 feet. That is 15 square feet or 1.39 square meters. So the maximum input power available, based on that standard number of 1,000 watts per square meter at the earth's surface, is 1.39 X 1,000 = 1,390 watts. The claimed output power is 3,000 watts. So the efficiency is 3000/1390 = 2.16 or 216%. That's impossible. Efficiency can never be greater than 1. You cannot get more power out than you put into an energy conversion system. I just saved my self from making a very bad investment!

Mark George is an American expat who has been living in Thailand since 2002. He's not working, but enjoying a (very) early retirement, made possible by smart money management. He also has a keen interest in Thailand's booming medical tourism industry where world class health care is available at a fraction of western prices. If you would like to learn more about world class health care at a fraction of the price visit The Medical Travel Site .


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