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Guide to Buying Solar Panels

The market is filled with so many options for solar panels.  The purchasing process can be a little confusing with different shapes, sizes, outputs, prices, and qualities to choose from. Many consumers find themselves choosing between saving money or going for quality. The basic principle behind the decision is simple: maximize the electrical output and life expectancy of the solar panel, while minimizing the cost of installation.

With this in mind, let’s take a look at the four most important factors to consider.

Power Ratings and Efficiency

Solar panels come with a data sheet which lists the specifications of the panel. One of those specifications listed is the warranted power rating, which represents the true amount of power (watts) that a solar panel is capable of right out of the box. This can be referred to by different names such as peak wattage, peak power, and maximum power (Pmax). This rating is determined by testing the best samples of solar panels from the manufacturer’s batch.

Also found on the data sheet is the tolerance rating. The true power output of the solar panel will be defined with a wattage range, using the tolerance rating.  The peak tolerance rating for solar panels is the most power it can output. Whereas, the negative tolerance rating is how much lower the panel’s output could be. For example, a panel that is rated at 200w with a negative power rating of 10% has the range of 180w-200w, but only 180w is guaranteed. You want a high number for peak tolerance rating, but with negative tolerance rating, a lower number is better. They are both giving the same information, just using different numbers.

Quality brands use mostly high-quality materials, and as a result, their solar panels have a lower negative rating. A lower negative rating usually translates into needing fewer panels. Using these ratings helps in determining whether to spend more money for higher quality solar panels. For example, you might find that buying more expensive panels with a lower negative rating will save you money overall. If you have two 200w panels – panel A with a negative power rating of 5% and a $315 price tag, and panel B with a negative power rating of 10% and a $300 price tag, you would need more B panels than A panels to reach your desired kW/h. Building your system using A panels (the more expensive, more efficient panels) would translate to higher initial cost, but lower lifetime cost.

Monocrystalline vs. Polycrystalline

There are two types of solar panels on the market: monocrystalline and polycrystalline panels. Monocrystalline panels are made from a single ingot of silicon, which is a more expensive process but also yields higher efficiency panels. Several smaller cheaper ingots are used to manufacture polycrystalline panels, which is a process that ultimately cuts costs but lowers efficiency. These cost savings are not usually passed on to the customer, so you should always look for monocrystalline panels.


Solar panels usually go through two testing procedures. One is performed by the manufacturer and one is performed by independent laboratories. The factory testing result is called the Standard Testing Conditions (STC) rating and the independent testing result is the Photovoltaic USA Test Conditions (PTC) rating. The PTC ratings are usually lower than the STC because they are tested using real-world conditions to measure the output of solar panels. They are therefore more accurate for knowing how a solar panel will hold up. For example, a solar panel might have an STC rating of 200w and a PTC rating of 150w.  Add to this a 10% negative power rating, and you’re essentially buying a 130w solar panel. When purchasing solar panels, go for a product that gives the PTC rating so you can be sure to get the true wattage needed.

Established Manufacturers

The lifetime expectancy of a photovoltaic panel is roughly 25 years with degradation over time. You want to look for solar panels that come with a warranty for at least 20 years. Performance warranties typically guarantee the output at 90% for 10 years and 80% for 20 years. Product warranties cover manufacturing defects of the solar panel, generally for 10-12 years. Most manufacturers offer some sort of warranty, however, not all manufacturers have the name brand to back that warranty.

When purchasing solar panels, you want to do your research and purchase from an established manufacturer that has proven itself to be reputable and is expected to be in business for a long time. Otherwise, you risk losing your warranty ahead of the 20 year period, in which case, your long-term investment costs could rise. This might mean that you have to spend a little more up front, but the additional long-term costs are minimal.

Find out more

Still, have unanswered questions? Contact us and find out more about maximizing your ROI when purchasing solar panels.




Image credit: Sundance Solar

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