Once again, life has shifted for my family of four and we are looking for our next home. While the home’s square footage, layout, and yard space are considerations, the feature I tend to get most excited about is where we can mount our next PV array. We are considering a home that has a large garage roof. Its excellent southern solar exposure and the lack of pesky vents and dormers to work around has got me thinking again about PV array design—and module selection. There’s ample mounting space, which could accommodate increasing the array size to not only meet 100% of our electricity needs annually, but also support a future electric vehicle.
There are a lot of modules to choose from (see the “Online PV Module Guide” sidebar) but there is also uncertainty about the survival of many PV module manufacturers because of the large supply-versus-demand imbalance that currently exists, along with the challenge for many manufacturers to produce their products at today’s cheap module pricing. These factors—combined with the usual considerations, such as module output ratings, power tolerance, efficiency, and pricing, along with new inverter, mounting, and module-level electronics options—make module selection more complex. To help me wind my way through the choices, I’ve devised a list of top considerations that will be helpful for any array design exercise. Additionally, a module selection example is provided, given our potential roof space and energy generation goals.
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.
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.