With blistering enthusiasm and a huge green bee in your bonnet, you’ve begun your research into solar and you’re doing all the right things.
You have researched multiple suppliers, spoken to your neighbours about their system and experience, and you’ve scoured the web for reviews and solar info. You’re starting to feel a little more clued in.
Now, you’re up to the part where you are starting to compare solar product performance. You want to know the best solar panels and associated products you can get for your circumstances and budget.
Of course, you are greeted with an ugly list of confusing numbers, and all of a sudden, the ‘blistering’ has all but gone from your now waning enthusiasm.
Not all of us are blessed with an innate affinity with numbers. Moreover, numbers that come attached to scientific terms and technical data leave us feeling out of our depth.
For inspiration, you quickly reference your last outrageous electricity bill. They’re numbers you understand. It doesn’t help, you’re feeling even more deflated, but you still refuse to accept outrageous electricity costs.
You have a choice.
1, you can blindly trust a sales pitch and purchase a brand-new solar array without any understanding of the critical numbers. Not wise at all.
Or 2, read this article for a user-friendly introduction on how to understand what makes one solar panel better than another. The wise choice.
Hopefully, you will see that these performance indicators are really quite accessible and relatively straightforward to understand.
On your research journey, you may have heard the odd solar boffin tell you that not all solar panels are created equal. They’re right. In fact, as solar demand has grown, more and more products and dealers have entered the market.
There are now a large number of brands and manufacturers from which to choose, and just like any other product or service, quality of components, performance, and workmanship will vary significantly.
Some high-end products may have incredibly high performance; however, they usually come with a matching price tag. Extremely high performance may simply not be necessary for your needs.
Other products might seem to be adequate in terms of performance, yet are suspiciously affordable. Products that are cheap are cheap for a reason. Compromise has been made, or more frighteningly, corners cut.
As a further variable, different solar products are more (or less) suitable for different applications, locations or circumstances.
A good analogy is: a Ferrari is one very expensive, high-performance supercar. However, if you need a car for driving across the desert, the Ferrari is just about useless.
Your home’s environmental circumstances might require you to purchase a particular type of solar panel technology to get the best electrical output.
It’s all about horses for courses, as the old adage goes. Understanding the numbers will assist you to in choosing a solar system with performance specs that are right for your needs and budget.
Reading A Solar Panel’s Data Sheet
Have a close look at this solar product data sheet. We’ll use this example to start getting a grip on the numbers. We won’t address every number here, just the critical ones.
OK, you’ve had a quick glance and you’re not excited in the least. Let’s break it down a little and put some meaning into these terms, charts and figures.
Look at the chart that says Electrical Specifications (STC). To ensure a set of industry standard performance numbers, solar panels are tested under specific conditions. That’s the (STC) bit, or Standard Testing Conditions or Criteria.
There are many factors that impact solar panel efficiency. Temperature, wind, aspect, load, elevation, to name just a few, and they’re all variable.
To ensure we know accurate nominal performance figures (will explain nominal shortly) of a panel, these variables are removed and/or standardised to arrive at predictable figures.
The chart directly underneath is labelled Electrical Data (NOCT). NOCT stands for Normal Operating Cell Temperature. These are the same measurements as in the STC chart but with adjusted criteria.
The new criteria are included to provide a more realistic impression of operational conditions. That is, some variables that reduce performance are assumed in this calculation.
These are the figures you are more likely to see while your panel is in operation, hence the better figure from which to make your performance assessment.
Remember we said, “nominal performance figures.” Nominal in this case simply means that it is impossible to provide a set of accurate or actual operating figures. When installed at your house, the panels will be impacted by local environmental factors.
Actual performance figures cannot be assessed until installed at your house. And, as environmental factors are in constant flux, there will be continual (usually slight) variations in the performance of your panels.
Electrical Specifications Chart (STC)
For the sake of standards, let’s use the Electrical Specifications chart (STC) to go through the numbers. This chart has the specification for different capacity panels or modules.
We’re looking at the numbers for the GCL-P6/60-245.
The first row is Peak Power Watts Pm (W). This is the maximum power this panel will produce, in this case, 245W. The higher this number, relative to the size of the panel, the better the panel’s performance.
The second row is Power Output Tolerance Pm (%). Ultimately, your panel may produce a power output that is different to what is specified on the chart. In this example, the panel could produce from 0% to 3% more power than is specified. This is a good figure.
If the tolerance is zero to positive (greater than zero), this is good news. Some panels have a tolerance that may go in to the negative, that is, produce slightly less power than is specified in the data.
The third row is Maximum Power Voltage Vm (V). This is the voltage measure at which maximum power is generated.
The fourth row is Maximum Power Current Im (A). This is the maximum power (measured in Amps) that can be generated.
Open Circuit Voltage Voc (V), row 5, Is the voltage the panel produces unaffected by load. That is, when there is nothing connected to the panel drawing power.
Row 6, Short Circuit Current Isc (A), is how many amps are being produced by the panel without any load connected.
Row 7 is important, Module Efficiency. You will notice this displayed in a prominent position on the first page. The module efficiency in this case is 15%. This is a good figure and relatively standard. The higher this number, even in small increments, the better.
Don’t be put off by the low figure. Yes, if your son came home from school with a mark of 15% in his maths exam there’d be questions, sternly directed. But in terms of converting the sun’s rays into electricity, a 15% conversion rate is very good.
NOTE: The fine print underneath the STC and NOCT charts explains the standardised conditions under which the panels were tested.
Let’s look at a few of the entries in the mechanical data chart.
The solar cells, in this case, are Polycrystalline (aka multicrystalline). While monocrystalline cells are generally considered more efficient, efficiency advances have been made in polycrystalline PV cells.
There is also a significant premium to pay for monocrystalline PV cells. Ultimately, depending on your environment, you may well get better value using polycrystalline panels and avoid a premium outlay.
The module dimensions in row 3 represent a reasonably standard panel size. The 60-cell configuration (row 2) is also quite standard.
Depending on efficiency figures, and generally speaking, the smaller the panel the better.
Weight will be an important factor for many. Roof structure will need to be considered when deciding on panels. Your consultant will assist here. A weight of 20 kg is at the upper end of the standard panel.
A more important factor here will be the right advice about the panel weight that will be most suitable for your roof structure.
Temperature coefficient is an important figure, particularly in Australia. That’s why we’re giving it its own heading.
Contrary to what you may logically assume, high temperatures, which are everywhere in Australia, reduce the efficiency of solar panels. Yes, we know it’s crazy, a device that is completely reliant on the sun to function stumbles in the face of too much heat, go figure.
Have a look at the chart titled temperature ratings. Note that there are 4 ratings on this chart. Look to row two column two, Temperature Coefficient of Pmax. This is the important number for your deliberations.
In this example, the number is -0.43% / °C. Essentially it means that for every degree (at the panel, not air temperature) above 25°C, the panel loses 0.43% of its efficiency.
In solar panel language, ‘hot’ is considered 25°C and beyond. It’s important to note that while it might be 25 degrees outside, at the panel, it could be as much as twice that.
So, if it is around 40 degrees at the panel, your panel, in this example, will be operating around 6% less efficiently. Remember, this is a pretty good number.
The best panels have a Pmax temperature coefficient of around -0.3% / °C. Panels that hit -0.7% should be completely avoided.
-0.43% is a good number, but the lower this number the better. The higher the number, the less efficient your panels are on the hotter days.
The maximum ratings chart provides maximum operational figures, or, the absolute limits of your panels.
Maximum Operational Temperatures (row 1), of -40 to +85°C is quite the tolerance. Remember, once you reach either end of these figures, your panels are still ‘operational’ but they’re not really producing much electricity.
The Maximum System Voltage (row2) is 1000 volts DC. That means the combined voltage of all the panels in the array is 1000 volts DC.
Interestingly, the domestic solar system limit in Australia is 600 volts. It would take between 18 and 20 of these panels operating at full efficiency to break the 600-volt Australian limit. The 1000-volt DC limit is simply a system capacity and unlikely to be relevant to you.
The Maximum Series Fuse Rating in this case is a 15-amp fuse. This fuse protects the system from overload and therefore damage.
Yes, warranties warrant a heading of their own also. After all, it’s a protection of your investment. The product warranty protects you against failure due to workmanship or defective components.
At 10 years, the product warranty in this example is good. While longer warranties are available, they’re not as common and generally attached to a pretty pricey product or cost extra.
You will come across panels that are warranted for as little as 5 years. Avoid these products. Clearly, the manufacturer has little faith in the product’s staying power.
It should be noted that this warranty comes from the panel manufacturer, not the installer.
The 25 years linear power warranty, or, performance warranty, refers to a guarantee of a particular level of performance over time. In this example, the time period is 25 years.
This warranty chart demonstrates that you are guaranteed that your panels will be operating at no less than 80% of the efficiency of the brand-new product at 25 years.
You will notice the comparison to other brand warranties (grey shading). Where the example product (Green shading) has a linear decline, the comparison warranties have a stepped decline.
So, in this example, after 10 years of the warranty, you are still guaranteed 90% of original efficiency, where the comparison warranty drops to 80% after only 10 years and stays at 80% for the following 15 years of the warranty.
Clearly, the gradual or linear model provides the best protection.
We understand that this sort of data can be a little uninteresting and chewy, particularly when all you want is cheaper electricity, not a science education.
Nonetheless, having been introduced to these terms, you will at least have some idea about the numbers when the solar sales consultant puts a panel product data sheet in front of you and says, “How good is that!”
Now you can effectively quiz the clever consultant about panel efficiency, temperature coefficient, peak output tolerance and why they’re trying to sell you monocrystalline panels.
You can also say to the consultant, “So tell me about the performance warranty…”, with a knowing grin on your face.
Knowledge, just like solar, is power.
If you want to see how much solar or battery storage could save you over the next 5 years, then take our solar saving calculator quiz below!
Or talk to an Instyle Solar expert about the best solutions for home energy storage or PV-panels.
Otherwise, head back to the solar blog to find even more great educational content.