Photovoltaic Tutorial:

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PV Module Specs and Features

Even though they all sort of look alike, you have lots of choices when it comes to choosing a photovoltaic module brand. While off-grid scenarios have more at stake in getting the actual size and features right, everyone benefits from a quality product. For example, some modules are more efficient than others in hot weather. Others are specially designed to work around the problem of partial shading. Some modules are manufactured with smaller length and width dimensions, so you can fit more of them in a limited space. And some modules are priced so low that's it's easy to forget about the specs entirely, which is not a trap you want to fall into under any scenario. Your motto here should be "Buy smart, not cheap."

Understanding the Basics

All solar modules possess an electrical capacity that can be measured in amps and volts. The basic unit of power, the watt, equals volts multiplied by amps, or P = V X I. (The "I" stands for current intensity). As mentioned earlier, each module contains several dozen solar cells that transform sunlight into watts. The more cells a manufacturer can pack into a module, the denser and heavier it is, and the higher the total wattage. This is measured in square meters (w/m2), a metric that was explained in the last section of the tutorial.

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Numerous companies make solar modules. They include the big boys, like Sharp, Sanyo, Sunpower, Yingli and Sun Edison. But there are also more specialized companies like Canadian Solar and Kyocera churning out quality solar panels as well.

Key Specifications

The primary considerations for choosing the right module include:

Here's the rundown on the most common specs you'll be checking out as you decide on the right product to get the job done:

Output in Watts – Most modules are sold according to how many watts of energy they generate at a standard irradiance level of 1,000 w/m2, and at a temperature of 25 degrees C (77F). The watt rating is determined by an indoor factory test known as the Standard Test Conditions value, or STC. For example, a 235-watt panel consists of solar cells that, when added together, total 235 watts. For example, if 60 cells comprise the module, then each cell tested at 235 / 60, or just under 4 watts.) The Output Watts or STC is also known as the nameplate wattage.

Low-wattage modules are not very densely packed with solar cells, and are relatively inexpensive. Those with a greater density of solar cells packed together, like 300 or 370 watts, cost a lot more. This is significant, because most residential roofs and backyards don't give you a lot of room to work with. If you can only squeeze ten panels onto a southern facing roof, and each is 240 watts, for example, then your system will be 2.4 kilowatts in size. (The actual output will be less than the nameplate watts, as you'll see in a moment.)

Some, though not all, PV module manufacturers also provide a PTC (Practical Test Conditions) rating, which is less than the STC. This measurement is more useful than the STC because it's conducted in real-world, outdoor conditions. (It's important to note that some tax rebate programs require a PTC rating to qualify for the incentive.)

Sharp Solar

Module Dimensions – Since all your modules will have to fit together within the space available, the length and width of each one must be factored into your design. Most standard modules are approximately 5 1/2 feet by 3 1/2 feet, but dimensions do vary. Some monocrystalline modules are smaller, however, and can be used when space is limited. In either case, to determine how many modules can fit on a rooftop or other location, you'll have to divide these dimensions into the area dimensions of the array space. Usually, designers divide the shorter module dimension into the area side-to-side dimension, which translates into a standard portrait configuration for the array. However, it's sometimes possible to squeeze more modules into an array area with a landscape orientation. Here. you divide the shorter module dimension into the roof's top-to-bottom area dimension. Check out this example of the calculation made with an Excel worksheet.

Operating Temperaure – If you experience extreme temperatures where you live, this spec is important. A range of -4°F to + 115°F (-20°C to +46°C) is typical for most modules on the market today.

Other Specs

Here are a couple of less critical but highly useful specs you may need to consider into system sizing formulas:

Power Tolerance – This spec, provided as a percentage, is an electrical performance range that takes into account the variable ambient temperature around the modules. For example, a 180-watt module with a tolerance of +/-5 % will produce between 189 watts to 171 watts depending on whether it's a hot or cold day. Obviously, a module with only a positive number is better than one that includes a negative rating as well. Alternatively, you can select a much cheaper module with a negative power tolerance, then compensate for the lower power output by designing a system with more overall STC watts.

Temperature Coefficient – Although it sounds counter-intuitive, high heat causes a decrease in voltage and therefore less power. (Remember, it's the light that creates the electricity, not the heat.) A module's temperature coefficient lets you know the maximum decrease in power output you can expect from the product on a hot day, and the power increase you can expect on a cold day. Coefficients are typically listed for power, voltage and/or current, as shown in the Sharp spec sheet above. The coefficient is multiplied by the difference in degrees (Celsius) between the STC rated temperature of 25°C (77° F) and the actual temperature you can expect at either extreme.

For more info on how coefficient values are calculated, see System Sizing in the Steps to Going Solar section.

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If You Live Where It Gets Hot...

Some modules are specifically designed to be more efficient in hotter climates. If you live where it gets hot often, you should consider one of these products. Click here for more info.

Besides picking a module with a low or no negative power tolerance, and a low temperature coefficient, you should consider implementing some form of heat mitigation when designing the array. For instance, if you install your modules four inches or more off the roof, this allows air circulation beneath the array, which has a net cooling effect. Placing gaps inside module rows also helps the air circulate around the array. Some resourceful homeowners have even incorporated a solar water heating system beneath the array to capture all the heat radiating from it.

Installing modules several inches off the roof allows cooler air to circulate and is now a standard practice in residential solar.

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More Specificiations

Efficiency – A solar module's general "efficiency" rating is the percentage of sunlight that gets converted by a module into electric energy. For example, if the module creates 100 watts of electricity when struck by 1,000 watts of sunlight, it's efficiency rating is 10%. Average efficiency ratings these days are 14-18% for modules, which is not very good. Future models are expected to show much higher percentages, but in the meantime you should look for as much efficiency as possible without having to pay a lot of extra bucks. If you have limited space for an array, you may have no choice but to buy higher priced modules in order to generate all the watts your PV system requires.

Voltage – Voltage is not a critical consideration in choosing a module for a grid-tied solar electric system, but you'll need the spec in order to choose a central or string inverter. On the other hand, off-grid systems that rely on battery banks must generally run on a 48-volt circuit or less so may require a 12-volt module, rather than the 30 to 40 volts on most grid-tied modules.

Utility interactive (i.e. grid-tied) systems that don't use batteries will run more efficiently on a circuit of three or four hundred volts. This means that a module rated at 30 volts might be wired in series with nine other modules to get a total operating voltage of 300 DC volts. The high voltage translates to less voltage drop and current losses as the electrons make their way downstream. Grid-tied inverters can usually accommodate a range of 150-500 volts. U.S. building and electric codes limit maximum voltage to 600 VDC. In Europe, it's 1,000 VDC. Note: If your system will incorporate microinverters, this calculation becomes mute.

Semi-conductor Type - You're more likely to come across this feature in the product sales literature than on a spec sheet. But it doesn't hurt to understand a little about the source material used in modules, tiles and other PV mediums. Here are the four major classifications:

Monocrystalline solar cells started out as silicon wafers sliced from a cylinder shaped ingot at the manufacturing plant. The cylinder itself is composed of material from a single seed crystal, and this molecular arrangement makes for the highest possible conductivity. That's why you get the greatest efficiency (i.e. watts per square meter) from modules packing monocrystalline cells. While it's the most expensive choice on the market, if you're smart about your design you may be able to use fewer modules. Conversely, some monocrystalline modules are manufactured at a smaller size, so you can fit more of them in a limited space.

Most residential PV systems use polycrystalline solar cells, due to a much lower module price. The unused, excess cuttings of the monocrystalline manufacturing process are melted down and used to make polycrystalline cells (also referred to as multi-crystalline).

Amorphous silicon is a non-crystallized, thin layered silicon that's designed to absorb more light as a result of its atomic structure. When you stack several layers together, you get sufficient power for a PV array. Also, the use of plastic and metal substrates surrounding the solar cells allows for more flexibility than standard modules. Amorphous silicon performs better in shady conditions or higher temperatures, although it's less efficient in terms of watts generated per square meter. It's often used in Building Integrated PV applications (BIPV), where the solar array becomes part of wall, roof or other architecture. For more on this technology, click here.

At left, the less densely packed random structure of amorphous silicon allows for greater proton absorption in a PV cell. At right, a diagram of a typical thin-filmed cell.

Thin-film solar cells may come packaged in a roll and can be laid out over a rooftop (like a rubber mat), without the need for rails and racks. Alternatively, they can be manufactured as roof tiles. You see both types from time to time on residential roofs. Although inexpensive both to buy and install, thin film solutions are less efficient in generating power than standard modules. A variety of materials may be used to make the cells, including amorphous silicon,  cadmium telluride, or copper indium gallium selenide.

For more insight on the various semi-conductor materials used to make solar cells, visit the website

U.S. Tile
Homeowners who need to replace an old roof might save money by using thin-film solar shingles.

Finally, here are a few more specs you may run across when shopping around for PV modules:

To learn more about module specs, see You can also try your hand at intepreting this product sheet for a Sharp 235-watt multipurpose panel. In addition, has an article, Understanding PV Module Specifications, which delves further into the subject.

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Next Topic: Array Orientation and Placement

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