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SolarTown Learning

Consumer Guide to Buying Solar Panels

10-01-2009
Category: Solar Panels

So you've decided to make the switch: Go green, or more aptly- go solar. Good for you. But where do you begin? At first glance, there seems to be a cornucopia of solar modules or solar panels on the market coming in various shapes and sizes, outputs, prices, and quality of materials and it can be very confusing. So how do you know which photovoltaic panels are right for you? Do you need to buy the most expensive photovoltaic (PV) modules on the market? Or are conventional generic solar modules sufficient for my needs? Your goal in purchasing solar panels is to maximize electrical output and reliability over their life expectancy while minimizing cost of installation. So how do you do this? What are the questions to ask before deciding on solar modules for your home?

These are some rules of thumb when shopping for home solar power panels:

  • You want negative power ratings to be as low as possible, no greater than 7%
  • Higher efficiency home solar power panels take up less space per amount of output, and are often the same price as less efficient panels.
  • The PTC wattage ratings are a more standardized and trusted rating of actual outputs from home solar PV panels.
  • Buying panels with from manufacturers with proven records offers more validity to the warranty and helps you protect your investment.

Minimum Warranted Power of the Solar Panel

The minimum warranted power of the solar home panel is the maximum amount of output that your panel is guaranteed to produce and the most important thing to consider before purchasing PV panels. Because home solar panels are created in batch, the manufacturer tests the best samples from the lot to determine the peak wattage. The rest of the batch may not test to such high ratings, so the manufacturer knows that the batch on the whole will produce within some range of the peak wattage.  Based on the quality and clarity of the silicon used in production, this loss is anywhere between 5-12% variance. That means if you buy a 200w panel and it has ‘negative power rating' of 10% the home solar panel is only guaranteed to produce 180w! Higher quality name brand manufacturers use higher quality materials and as a result try to maintain a negative power rating no greater than 5%.  As a rule, try and keep the negative power rating below 7%. Any greater loss than this and you will need to purchase more solar modules to offset the energy loss when calculating your energy needs based on the name plate rating.

Let's take an example.  You determine your energy needs to be 20KWH a day. At 5 hours of direct sunlight a day, you need a system to produce 4Kw per hour. If you are looking at two different nameplate 200w panels for installation, Panel A has a negative power rating of 5% and Panel B 10%.  Panel A is guaranteed to provide 190w while Panel B, meanwhile, only guarantees 180w. Therefore, you would require 22 Panel As to meet your needs and 21 B Panels. It does not sound like a big difference, but what if the cost of Panel A is $700 per panel and Panel B is $715 per panel? You would save $400 dollars by buying the Panel B because it has the lower negative power rating! And, there is a good chance that panels constructed of higher quality silicon are producing more power than the minimum guaranteed.

Higher Efficiency Home Solar Panels

Generally speaking, the higher the quality silicon used in production of solar panels, the higher the efficiency. The higher the efficiency, the more power per square inch the panel is producing. High efficiency panels are cut from large, expensive, single ingots of silicon, and are referred to as mono-crystalline.  Commodity or lower cost photovoltaic panels are often manufacturer from several smaller cheaper ingots, and are known as polycrystalline, to cut down manufacturing costs. This savings is rarely passed on to consumers, and they end up paying the same price as higher efficiency panels.  It is also possible to correlate higher negative power loss with less efficient cells, costing you more money at the time of initial investment.

Look for PTC Ratings

It is important to understand the difference between Standard Testing Conditions (STC) watt ratings versus that of the independent laboratory ratings called the PTC rating. The PTC (PVUsa Test Conditions) rating is required by many states in the US of PV panels before they can enter the market for reasons of consumer protection. The Standard Testing Conditions are performed by the manufacturer, and are often times inflated to market the PV panels at a higher price. The PTC rating is the superior benchmark as its independent status gives it no motive to present bias.  Following the same example as before,  if Panel A and B both have an STC rating of 200w but Panel A has a PTC rating of 150w and Panel B has a PTC rating of 200w, clearly panel B is a better deal.  At these rating discrepancies, if you purchased A Panels you may need to purchase several more panels than if you'd gone with option B, costing thousands of dollars in initial investment.

Warranties from Established Companies

Durability and lifetime reliability are key components of home solar panels.  The typical lifetime expectancy of PV panels is 20-25 years or even more, and most PV manufacturers nowadays come with lifetime warranties against manufacturing flaws and output defects  for at least twenty years-and  20 years should be the minimum when shopping for warranties as you'll want to calculate your return on investment over a time period of this length.

What not all solar panel manufacturers have is a proven track record of reliability and brand name backing that add value to the warranty. Many companies in the solar industry have come and gone over the years, and it is only through the producer of your panels that your warranty is valid. What happens if 5 years after you purchase your home solar PV panels, the company you purchased from goes out of business, and 5 years after that your PV panels start to break down? There will be no one to honor the 20 year warranty, and you'll be out of luck! The safest way to protect your investment is to stick with conventional high performance technologies produced my manufacturers with proven track records. While it may be cheaper to purchase alternative commodity panels, it does not seem like a worthwhile trade-off to jeopardize the life span of your solar investment for small marginal savings. It may cost you money in the longer term.

Other Rules of Thumb

Now you should have a better understanding of what to look for in solar panels and why it is important to you, the consumer.  As you better familiarize with the photovoltaic market you may find you have additional questions regarding the differences of solar panels, and SolarTown would be happy to help answer them. Please feel free to contact us or post your questions in the Ask an Expert section. And if you have any suggestions for other rules to follow for consumers, please share them with us on our Facebook page.

Tags: home solar panels, solar module ratings, wholesale solar panels, discount solar panels, solar modules, photovoltaic panels, PV panels, PV module, consumer guide, crystalline silicon panels, photovoltaic panels, PTC ratings for solar panels, STC ratings for solar panels 

Updated: July 20, 2010


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