Solar Panel Selection For Grid-Tied Residential Systems
Selecting a solar panel is one of the most important decisions you will make when designing a solar PV system, but with the huge number of different panel types, technologies, sizes and capacities currently available, it can seem impossible to select the right one for you.
To help you with this, we have prepared this comprehensive guide, which covers all of the main points you need to consider, and will help you select a solar panel that is appropriate for your situation, and that you will be happy with for years to come.
Solar Panel Types
One of the first things you will notice, is that solar panels come in a variety of different technology types. Each has their own benefits, and all can be suitable for residential solar systems. The three main groups currently are poly-crystalline silicon, mono-crystalline silicon, and thin film.
Mono-crystalline panels are generally more efficient, that is, they are able to convert more of the sunlight that hits them into usable electricity. They also perform better in higher temperatures, but are usually more expensive.
Poly-crystalline panels are typically slightly less efficient than mono-crystalline panels, and therefore will take up more roof space for the same amount of power, but they also usually cost less per watt (W) (This is discussed further in the next section).
Thin-film panels have the lowest efficiency of the three, so need the most area, but can be the cheapest. They are also much thinner and lighter than crystalline panels, which can make installation easier.
Currently poly-crystalline are the most common panels for residential installations as they tend to provide the best balance of cost and efficiency. However where roof space is limited, mono-crystalline can also be a good choice. Thin-film solar panels are currently not very common in rooftop PV systems due to the large area they require.
Cost and Efficiency
As mentioned above, some types of solar panel technology are more efficient at converting sunlight into electricity than others. It is important to know that there can also be large differences in efficiency between solar panels of the same type and size. This can be due to the quality of semiconductor material used in the cells, as well as several other factors, such as the number of busbars they have, or the type of anti-reflective coating used on the glass surface.
There are obvious benefits to using higher efficiency solar panels: You can either use less panels to reach your desired system capacity (which reduces mounting structure and installation costs); or you could install a system with a higher rated power using the same number of panels. However, when considering the efficiency of solar panels, it is important to also consider the cost.
As you would expect, lower efficiency panels are cheaper, and higher efficiency modules are more expensive. However when you look at the cost of a module against it’s generating capacity, you will see that lower efficiency modules will often also be cheaper on a cost per watt basis. Therefore it may be cheaper to reach your desired capacity with lower efficiency panels, although it will use more panels, and therefore roof space, to get there.
The solar panel capacity which is the most appropriate for your PV system will depend on energy requirements, cost, and your available roof space.
For example, if you only have a small amount of roof space available or solar panels, but you have a high household electricity demand, then higher efficiency panels may be a better choice.
On the other hand if you have enough roof space to easily reach your desired capacity with lower efficiency panels, then this can be the better option due to their lower cost per watt.
60 vs 72-cell Panels
Crystalline Solar panels are not just one large piece of photovoltaic material, but rather are made up of a number of smaller sections of semiconductor material called cells. The number of cells is not the same for all panels, and the two most common numbers you will see in panels suitable for residential systems are 60-cell and 72-cell panels.
Generally, either 60-cell or 72-cell panels can be used in residential grid-tie installations, at around the same installation cost and using the same equipment.
Since 72-cell panels are larger, they are heavier and slightly more difficult to install. On the other hand, you need fewer of them, which means fewer cables and connections. These differences are minor and generally not worth worrying about in a residential system, unless difficult installation conditions favor a lighter panel.
One deciding factor between the two in a residential system may simply be which one fits the dimensions of the roof better.
On a roof where there is not quite enough room for an additional row of 60-cell panels, it may be possible to instead change to 72-cell panels to fit some extra capacity.
Aesthetics (What the panels look like)
So far we have only talked about the functional characteristics of solar panels, but how the panel looks can also be important to some home-owners. Each of the different types of solar panels have a different appearance, which may also sway your choice.
Poly-crystalline – The cells in these panels are made up of multiple crystals, giving them their speckled blue appearance. They have an aluminum frame and white back-sheet.
Standard mono-crystalline – These panels have been cut from a single crystal so have a more uniform look. They are also much darker than poly-crystalline panels. They also have an aluminum frame and white back-sheet.
All black mono-crystalline – These are mono-crystalline panels, which have been designed with appearance in mind. These panels have a black back-sheet and frame. Some may still have metallic busbars (the wires running on top of the solar cells), while others will also have black busbars giving them a totally black look. All black panels will typically be more expensive than standard panels.
Solar Panel Quality
While it may be tempting to select the cheapest solar panel you can find, especially where you have plenty of available roof space so are able to select lower efficiency panels, it is also important to consider the quality of the solar panel.
There are plenty of poor quality manufacturers out there who will charge very low prices but on very low-quality panels which will not last the 25 years you should expect from your system.
Three ways to judge the quality of solar panels are by looking at who the warranties on the panel, who the manufacturer is, and what independent tests have they passed.
One potentially overlooked item to consider when selecting solar panels is the manufacturer’s warranty.
It is important to understand that there are two different warranties on solar panels. The defect warranty, and the output warranty.
Defect Warranty (also “Product Warranty” or “Manufacturer’s Warranty”)
- This warrants the panel as well as cables and connectors that come with the panel, will be free from materials and workmanship defects and usually lasts for 10-12 years. Manufacturers will repair or replace any faulty panels within this time.
- It is important to read the wording of the warranty however, as some manufacturers will pay for the shipping and reinstallation of your solar panels, while others may only cover the replacement panel.
- The power capacity displayed on the datasheet of a solar panel is the amount of power the panel should produce on day one (under STC conditions). However, like most of the products we buy, solar panels will degrade with time.
- The output warranty gives a minimum power output the panels will produce each year for the expected lifetime of the panel. In typical solar panels today the guarantee is around 97% the first year (the larger drop in the first year is due to Light Induced Degradation (LID)). It then declines at a constant rate of around 0.7% per year until around 80% at year 25, when the warranty ends
- While 25 years is typical for an output warranty, some solar panels, such as glass-glass models, may have an output warranty as long as 30 years.
- Beware that these output warranties are often impractical to enforce for the average homeowner. For example, you have to uninstall your panels, you may have to pay to send your panels to get tested, and then you need to reinstall the new ones. This may not be worth the compensation you will get under some warranty schemes.
- As with the defect warranty, some manufacturers will pay for the shipping and reinstallation for you so it is important to check the warranty in detail.
Any warranty is only as good as the company that is offering it. It doesn’t matter how long the warranty is if the manufacturer goes bankrupt a year after installation.
Ideally, you should always use a reputable manufacturer. Their panels may be more expensive, but not only will the panels be more likely to last their expected lifetime, but the manufacturer is also more likely to be around to honor the warranty if something does go wrong.
Independent tests have been developed by organizations such as IEC (International Electrotechnical Commission) and UL to test solar panels for quality. Passing these is no guarantee that solar panels will last the 25+ years that is expected of them, but it is a good starting point.
Some of the more important standards you should look out for are:
(These standards can usually be found on the datasheet of the solar panel. If they aren’t listed there, you can check directly with the manufacturer.)
IEC 61215 (also EN 61215): Crystalline silicon terrestrial photovoltaic (PV) modules – Design qualification and type approval. This is one of the most important standards and consists of accelerated stress tests to evaluate the long-term performance of PV panels. You should avoid solar panels without this certification (unless they are not crystalline silicon).
IEC 61646: Thin-film terrestrial photovoltaic modules – Design qualification and type approval. This is essentially IEC61215 for thin-film solar panels.
IEC 61730: Photovoltaic module safety qualification
Part 1: Requirements for construction
Part 2: Requirements for testing
UL1703: Flat-Plate Photovoltaic Modules and Panels (Applicable in the United States)
Depending on which country or state you are in, there may be different laws and regulations around which solar panels you can install.
Some countries may require the panels to have a specific certification, others may have a list of approved panels. Still others may have a list of panels that must be used if you wish to qualify for financial incentives.
You should generally be fine if you are purchasing the panels from your local store, however if you are purchasing the panels online, especially from an overseas supplier, you should be certain they are legal to install in your region before making the purchase. If you can’t find the information online, another way to check is by contacting your local utility.
Do you have any questions or comments? Let us know in the comment section below!