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Key metrics for comparing solar panels: PTC solar, STC rating and temperature coefficient

So you’ve made the decision to go solar. Well done! If designed right, a solar power system is a great, long-term investment with both financial and environmental benefits.

Choosing the right solar panels is one of the most important steps in designing your solar power system. There are hundreds of options, and while all panels are designed for one purpose – to convert sunlight directly into electricity, not all panels are created equal. Different solar cell technologies and the quality of manufacturing can impact the performance of your panels. For these reasons, comparing solar panels and finding the right one to meet your specific system needs can sometimes be a complicated task; but, it doesn’t need to be.

In this article we’ll explain important metrics like PTC power rating, STC, degradation rate, efficiency and temperature coefficient.

PTC and efficiency: key metrics that measure panel quality and performance

Efficiency – Efficiency measures a panel’s ability to convert sunlight into DC electricity. The higher the efficiency, the less area needed to generate a given amount of electricity. Panel efficiency will dictate the physical size of your system so it’s an important criteria if you’re a homeowner or business owner with limited space. From an environmental perspective, more efficient panels translate into less material and less manufacturing waste.

PTC Power Rating – Power ratings measure a solar panel’s power output in watts. Every panel is assigned a nameplate power output value, which is measured using STC (Standard Test Conditions). Because the STC rating is a measurement calculated under ideal laboratory conditions, it’s not always a useful metric for designing your system.

A more accurate measurement commonly used in the U.S. is the PTC (PVUSA Test Conditions) rating. The PTC measures panel output under more realistic conditions. In some states, the PTC rating is the benchmark for calculating system rebates so it’s an important metric to know. If a PTC value isn’t given, a good rule of thumb is that the PTC rating is about 10-15% less than the STC rating.

Warranty – Manufacturers provide power output warranties to ensure that their panel performance won’t fall below a specified level over the term of the warranty. Depending on the manufacturer, terms usually range between 20 to 25 years. For example, a manufacturer might provide a warranty to guarantee that peak power output does not fall below 80% for 20 years. Sometimes, manufacturers also include a secondary warranty that ensures a higher output level over a shorter period of time (e.g. 90% peak power output for first 10 years). Additionally, most panels come with a workmanship warranty that protects consumers against defective panel parts for the first 5 or so years of operation. Warranties can protect you against operating risks, so you should factor them into your evaluation of panel options.

Interested in learning more about solar panel quality? Check out EnergySage’s solar panel quality rating system.

Solar system maintenance: degradation rate and temperature coefficient

Solar Panel Degradation Rate – The solar energy degradation rate measures the rate at which a panel’s power output decreases over time. Typically, degradation rates for quality panels are below 1% / year. Ideally, you want this number to be as low as possible. SunPower solar panels offer the industry best degradation rate at 0.32% / year.

Temperature Coefficient of Power – The output of a solar panel is inversely proportional to the temperature of the panel. In other words, as temperature increases, panel performance decreases. The Temperature Coefficient of Power quantifies this relationship between heat and power output. It’s expressed as: % Power / °C. The smaller the temperature coefficient of power, the better the panel will perform at higher temperatures. If you happen to live somewhere hot, a low temperature coefficient will be important in maximizing your system’s efficiency.

Manufacturer Background – It’s also important to know something about the panel manufacturer. A financially stable company is more likely to honor its warranties if your system fails. Additionally, you can expect that a well-rooted company with a strong reputation will provide more customer support and be more open to customer feedback. For those that like to buy American, you’ll want to understand where the panels and components are being manufactured. Doing some homework on the manufacturer is a great way to lower the risk of your solar investment and to ensure that all of your goals are being met.

Panel Aesthetics – If the look of the panel is important to you, always request the spec sheets for the models you are considering so you can see a picture. You might also check out our case study section to see how various panels look on different roof types. Panels come in a variety of colors (black or dark blue usually, but new colors have hit the market recently) and the casing may also be a different color than the cell area.

The metrics outlined above will help you to evaluate the panels included in your solar quotes to maximize the return on your investment. For most smaller, residential installations, a deep investigation of panel quality differences is probably not necessary. That said, consumers should ask providers about the differences in the technologies they are suggesting since technology quality can impact your solar power system’s the financial returns. The measures above affect factors such as the amount of savings you will realize and the value of production-related incentives such as SRECs which in turn will affect your return on investment.

Even if you lease your solar system, your financial returns and your environmental benefits can be limited by lesser quality panels. Of course, there are other variables to consider beyond just the panels when making your final decision. Things like the price of your installation and the quality of your solar installer are also important.

How EnergySage helps you compare solar panel quotes

All of the metrics above will help you make decisions on the equipment you choose for your solar energy system. However, there’s another important factor to consider as you evaluate all of your options. Every solar installer uses a different set of assumptions to calculate the amount of electricity your solar panels will produce and the savings you should expect from your system. This means it can be difficult to make comparisons between quotes from different solar companies.

When you get solar quotes in the EnergySage Solar Marketplace, the information that each installer uploads for your quote is displayed in a standardized quote comparison table. The table uses a standard set of assumptions for every quote so that you can make true apples-to-apples comparisons of electricity production and 20-year savings across all of your offers. These assumptions include:

  1. Electricity bill data – Solar installers typically make some assumptions about how much electricity you’ll use in a year, as well as your utility electricity rate, when developing a quote for your home. These assumptions will impact your estimated 20-year savings with solar. To standardize your solar offers on EnergySage, all of the quotes you receive in the Solar Marketplace are based on the same electricity bill data.

  2. Electricity rate increases – Your utility electricity rate will increase over the 20+ years that your solar panels produce electricity, which is one of the primary reasons that solar is such a good investment. To calculate your 20-year solar savings, solar installers will make an assumption about how much your electricity rate will increase each year. Installers offering the same equipment at the same price could come up with very different 20-year savings if they use different electricity rate increase assumptions. The Solar Marketplace standardizes the electricity rate increase across all of your EnergySage quotes.

  3. Solar renewable energy certificate (SREC) values – If you live in a state that has an SREC market, you can sell the SRECs generated by your system for extra cash. However, the value of these SRECs will fluctuate based on supply and demand. Calculations of your 20-year savings on the EnergySage Solar Marketplace use a standardized SREC value across all of your quotes to make it easier to compare them.

  4. System degradation – Every solar panel system’s electricity production will inevitably decrease over time. Solar installers may use different assumptions about how much your solar panels’ electricity output will decline in a year. On the EnergySage Solar Marketplace, the system degradation rate is standardized across all of your quotes.

If you haven’t taken the first step to get quotes for your solar project, visit the EnergySage Solar Marketplace to quickly receive multiple quotes from our network of high quality installers. You can register even if you already have quotes from solar installers – our Solar Advisors will help you upload them into your account so that you can take full advantage of the Marketplace’s quote comparison capabilities.

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