Power tolerance is a measurement of how close a module’s actual output will be to its rated output under standard test conditions (STC: cell temperature = 77°F and irradiance = 1,000 watts/m2). For example, if a 200-watt module has a power tolerance of +/-3%, its actual output (under STC conditions) can vary from 194 W to 206 W. Some modules have a positive-only (such as “+5/-0”) power tolerance, which means that these modules should be able to produce at least rated power under STC, and possibly more.
Choosing PV Modules
PTC ratings (PTC-to-STC ratio) specify module power output for settings that more closely represent real-world conditions, which makes them lower than STC ratings. The STC temperature of 77°F for a module’s cells is often not a very realistic temperature for these dark cells exposed to direct sunlight; their temperature will commonly be much higher. As cell temperature increases, voltage drops, which reduces module power output. PVUSA test conditions (PTC) calculate module output using an ambient air temperature of 68°F (at 1,000 watt/m2 irradiance), which typically causes cell temperatures to be about 113°F to 122°F (36°F to 45°F higher than STC).
However, modules are sold based on their STC-rated power output rather than by PTC ratings, making it more difficult to compare realistic performance between modules. A PTC-to-STC ratio is included in the table for all modules. The closer the PTC rating is to STC, the higher the module output is under more common conditions. For example, if a “200-watt” module has a PTC-to-STC ratio of 0.9 or better, then its PTC rating should be 180 W or higher; if the ratio is 0.85, then its PTC rating will be only 170 W. Although that difference may seem negligible, when you add the power up for an entire array, it can be significant.
Module voltage and string inverter input window need to be considered for any grid-tied PV project that uses a string inverter. Each module has a specific maximum power point and open-circuit voltage, and each site has specific temperature ranges it will experience, which will determine the actual voltage each module will operate at.
Additionally, each inverter has its own input voltage limitations, which will dictate string size for module models being considered. Many string inverter manufacturers have online sizing calculators to help find string configurations that work for each PV module, considering local climate.
Power density of a module is dependent on module efficiency and is given in watts per square foot. The greater the density, the more power the array can generate per square foot. But higher module efficiency also means more dollars per watt, so before you assume you need a high-efficiency module, check the amount of space you have compared to the total power you want (see the “Selecting Modules for My Garage” sizing example).
Module dimensions need to be considered, especially if you’re working with limited mounting space but trying to maximize array capacity. Often, you’ll have to compare layouts, including both portrait and landscape configurations, to find the appropriate array layout for a rooftop. When using a string inverter, layout options may need to consider the required number of modules in series (and the number of parallel strings) to make sure the array layout is compatible with the inverter’s input and output limitations.
PV manufacturer location can be an important factor. First, some production-based incentives, such as Washington state’s RE System Cost Recovery program, pay a higher per-kWh incentive for systems with locally manufactured equipment. Systems funded by the American Recovery and Reinvestment Act (ARRA) and installed on public buildings must use domestically manufactured modules (or foreign modules that use 100% domestic cells). The less distance a module has to travel to its ultimate destination, the less embodied energy that module has. Finally, many people want to support local manufacturing jobs over foreign jobs and imports. See “PV Manufacturer Profiles” sidebar for information on how to find manufacturers offering ARRA- and Buy American Act-compliant modules.
Module frames and back-sheets are important to mounting technique and aesthetics. Options include frameless modules, module frames with mounting grooves for rail-less mounting, modules that allow some light to pass through (popular for awning systems), and dark back-sheets (black), which provide a more uniform look within the array.
PV wire leads are required for ungrounded arrays. Transformerless inverters are becoming increasingly popular because of increased inverter efficiency and enhanced safety (see “Ungrounded PV Systems” in HP150). However, they do require the array to be ungrounded, and the modules selected must have “PV-wire” cables for these installations. (PV-wire has specific benefits over standard USE-2 conductors including thicker insulation, higher voltage ratings, better UV resistance and flexibility in extreme cold.)
Warranty is important, and while most PV manufacturers offer 25-year power output warranties, material warranties can range from two to 10 years. A warranty is only helpful if the company offering it sticks around to service a future claim. With the PV manufacturing industry undergoing so much change right now, and many companies merging or exiting the market, some manufacturers are offering noncancellable warranties serviced by third-party insurance companies.
Cost is always a factor and budget can dictate the array you ultimately purchase. Module pricing has been on a downward trend over the last few years. A brief online search shows many modules are available for $1.50 per W; some even below $1.
While the array may cost less up-front, you may be without support should problems arise. In certain areas, installing a grid-tied system without a licensed installer means forfeiting some incentives. While the modules table lists more than 900 modules, no matter whether you buy online or through your local PV installer, available options will be limited to those modules currently offered by that supplier.