How to Read a Module Spec Sheet Header

2022 is promising a lot of changes in the solar module market. In past years, it was often realistic for a solar installer to choose one or two modules, load them into their quoting tool, and standardize on the same modules for long periods of time.

Things have changed. Supply chain disruption, stoppage at the ports, freight disarray, uncertainty about a bifacial tariff exemption, and the introduction of a new tariff petition this month have made the possibility of smooth sailing on your module choice low. Willingness to be flexible on modules will serve nimble companies well. In many cases, large installers have hedged their bets by buying up standard efficiency modules. This is why it’s now the case that premium modules are readily available when others are harder to get.

Even in “normal” times, understanding the particulars of a solar module’s data sheet can be critical. The salesperson or project manager who only looks at watts might be looking for trouble. For that reason, we are going to break down some important and useful pieces of information you should pay attention to on the module spec sheet, so you can ensure you choose a module that fits your company, your customer, your electrical needs, and your budget.


We often hear installers say that they want “the most efficient module” that they can find. But what does that mean, exactly? Often, they are talking about Pmax/W, which is the maximum output power of a module. It’s the “nameplate” rating of the module. Many equate a higher nameplate with higher efficiency. But the higher nameplate module may not be the most efficient module you can get, for two main reasons.

First, module manufacturers have been introducing new cell sizes, cell types, and module form factors. In addition to the more traditional 60 or 72 cell mod, most vendors now offer 66 cell or 78 cell modules. 54 cell modules that are shorter and wider are also being introduced. That difference is important. A 380 watt module that has 60 cells will have roughly identical efficiency as a 420 watt module with 66 cells.

Second, there are different cell types, sizes, and technologies. It’s beyond the scope of this article to dive into the differences. But it is generally true to say that a “standard” mono perc module with 66 cells will be less efficient than a 60 cell module that has HJT technology, or back contact, or additional buss bars, or other advancements.

For these reasons, look for “Panel Efficiency” on the spec sheet. The higher the number, the more efficient the module is. In the above example of a 380 watt 60 cell vs. a 420 watt 66 cell, the efficiency number will be the same or at least quite close.

At that point, you could then look at practical needs for your project. Let’s say you have a residential project, in which you can achieve the homeowner’s goals if you can install 9,000 watts on their roof. An inexperienced salesperson may assume that they need to choose a module with as high a nameplate rating as possible.

But consider this scenario: the customer’s roof itself is 18’ tall by 26’ wide. Module Option A is a 66 cell module with a nameplate rating of 420 watts. The module width is 41”, so you can fit 8 columns. But the module length is 75”. Three rows high would come in almost 19’ and exceed the available 18 feet. So you’d be limited to two rows of 8, meaning you can only fit 16 modules for a grand total of 6720 watts.

Module B is a more efficient module; lower watts at 380, but higher efficiency because those 380 watts are packed into a 60 cell. In this case, the width is the same at 41”, but the length is 68”. Three high would come in at just over 17’. Therefore, the “smaller” module in terms of watts can get you more watts on the roof – 9,120, to be exact.

In other words, don’t just look at the big number nameplate rating. Efficiency matters, and sometimes the “smaller” module can achieve more watts.

MLPE Compatibility

It often pays to pay attention to module compatibility with specific micro-inverters or rapid shutdown devices (For the sake of this article we’ll lump all of that into Module Level Power Electronics, or “MLPE’s”).

For example, it is often desirable to purchase fewer MLPE’s, by choosing a larger nameplate module. Let’s say a light commercial project has a goal of approximately 100 kW on a rooftop. A 450-watt module can achieve the goal with 222 modules. They’ll need to purchase 111 MLPE’s if using a 2:1 RSD device. Some large-format modules coming to market offer nameplate ratings over 500 watts. So if the designer can achieve similar project nameplate watts if they choose to install 188 modules at 530 watts. This would result in 94 MLPE’s, eliminating the cost of 17 MLPE’s.

So far so good, and we have seen many installers successfully value-engineer their projects using this method.

However, in addition to the efficiency considerations mentioned above (can the installer physically fit the larger 530’s on the roof space they have to work with?), they must take the additional step of looking at open-circuit voltage (Voc) and short circuit current (Isc). Assuming compatibility is a terrible idea and can lead to project delays and additional cost later. Better to call the sales rep of your chosen MLPE, with your module option spec sheets in hand. They’ll take your Voc and Isc data and confirm (or deny) compatibility. In many cases, the MLPE company already has a compatibility calculator on its website. You can plug in their requested data, and their calculator will tell you whether it’s a thumbs-up or down.


Frequently, an installer will position modules in a “better – best” sales strategy. “Better” would be a quality module from a reputable company, but it may have average efficiency and a less valuable warranty. Let’s dig into what might differentiate a “better” from a “best”;

Module manufacturers always provide information about their warranty on the spec sheet. There are several factors that you should look at when comparing one module to another.

First, each warranty will have a product/materials warranty. Generally, you will find product warranties for 10 years, or 12, or 25. This part of the warranty will cover defects such as delamination, water intrusion, faulty connectors or junction boxes, blown diodes, etc. During the specified timeframe, you can make a warranty claim with the vendor and get a replacement.

Second, each module will have a power output warranty or “linear degradation” warranty, represented by a diagonal line on the spec sheet. You should pay attention to a few things here. Each module has a step down from 100% for the first year. Usually, this is somewhere around 92 to 98% depending on the vendor. From there as a starting point, there is a straight line from the end of the first year to the end of the 25th year. What this is saying is that every silicon-based module will degrade over time, and the module warranty guarantees that the module will not dip below that diagonal line in the output. This is often the main differentiator between a standard module and a premium module… the premium will degrade slower. That can be a major factor for the homeowner. They may pay more as an item of up-front capital expenditure for a premium module, but over 25 years, that module will produce more total kilowatt-hours than a standard module, offering better overall lifetime value.

Third, a few module manufacturers have added labor coverage to their warranty. What this means is that if a module is determined to have a product/workmanship failure during the 25 years, has fallen below the warranted power output line, then the labor warranty will cover labor costs (up to a specified dollar amount) for the installer to go to the site and make the swap. Truck rolls can be quite expensive, especially for companies who operate in heavy-traffic metropolitan areas, or those who drive long distances in rural areas. Therefore, having the labor coverage that premium modules provide offers a major advantage to both the installer and the customer.


Other Considerations

While efficiency, MLPE compatibility, and warranty are the most frequently discussed topics when deciding on a module, they are not the only considerations.

For example, system designers often pay attention to the Temperature Coefficient of Pmax. This represents the max power of a module in terms of temperature and is always negative. It indicates the inverse relationship between power and temperature. The higher the temperature, the lower the output. This may make a big difference when designing a project in West Texas, for example.

The temperature coefficient of Voc is related, indicating a decrease of open-circuit voltage with increasing temperature. This factor is often used to determine maximum string sizing when using a string inverter.

From time to time we will get a request for a module that exceeds a certain Pa rating for top loading or wind uplift. High ratings are often needed in cold areas to ensure the module can handle snow load, or in coastal areas to ensure the module can withstand high winds. Most spec sheets will show these numbers. If they don’t, just ask the module rep.

Finally, there are factors that aren’t on every spec sheet that you may get a request for from time to time: One is an anti-glare study. If you are developing a project near an airport, you’re going to need this, and your module rep can provide it. Another is what’s called a .PAN file. This is a file extension of the PVSYST software that gathers all the electrical characteristics of the module and can use them to simulate the production of the PV array in the aggregate. Again, your module rep can send these to you upon request.

Ultimately, we are all in the business of maximizing value for our customers. Modules are the most visible part of the installation and usually the largest budget item for the project. For that reason, the decision shouldn’t be made lightly. Investing in a premium, top-tier module may cost more upfront, but can pay off in additional production for your customers while giving you the peace of mind that a long warranty from a reputable manufacturer provides. As always, our engineering professionals at Greentech Renewables are here to help you. Reach out to us today if you’re ready to begin your solar journey or need assistance with product selection. Please let us know your comments or questions below!


Brian Bakalyar commented 2 years 1 month ago

Well said, Brian. Attention to detail is critical when vetting module options and you've done a great job distilling key factors.

Buck commented 2 years 1 month ago

Why isn't there discussion on having panels on your roof and you need a new roof. Cost of removing panels, adding new roof and reinstalling panels ?

2 years 1 month ago
Written by
Brian Eisenbise