Cell Type & Efficiency. If you have limited mounting space, PV module efficiency is a key consideration. Modules with crystalline silicon PV cells—as opposed to thin-film—will likely be required, since thin-film modules produce about half of the wattage per square foot.
Within the crystalline module category, there are variances in power density (watts per square foot). Average power density is about 12.7 W per square foot, but some modules have higher power densities: Sanyo modules range from 14 W to 16 W per square foot; SunPower modules range from about 16 W to 18 W per square foot.
Using a module with higher power density means getting more power out of your usable mounting area. For example, let’s say our shade-free mounting area measures 20 feet by 10 feet, for an area of 200 square feet. Without considering module dimensions, choosing a module with a 12 W per square foot power density will yield a 2,400 W array; selecting a module that yields 18 W per square foot results in a 3,600 W array—a 50% improvement.
The downside is that modules with higher power densities typically are more expensive per watt (about 7% to 12% more). Decreased installation and racking costs for higher-efficiency modules may—or may not—amount to much. In most cases, if you have plenty of installation space, you won’t likely want the more expensive modules.
Bifacial modules that produce power from both the front and back of the module (Sanyo’s HIT 190 W and 195 W) report two values for module efficiency. These modules have clear backing, allowing some light to pass through, and can generate some energy from the reflective light that hits the back of the module. These modules can be good choices for awning and carport installations that can take advantage of reflected light.
Module Size & Dimensions. This will determine array layout—how many modules can fit in the available space. Differing roof planes, such as trapezoids (created by hip roofs) require carefully choosing modules with appropriate dimensions, so that roof space is maximized without hanging modules off the roof’s edges. Required setbacks for local fire department guidelines, accessibility for maintenance, module mounting infrastructure, and module interconnections and string layout will also influence what size modules will work in an array at your site.
Power Tolerance. This is the variance from the module’s rated output. For example, a 200 W module with a +/-5% power tolerance may produce anywhere between 190 and 210 W. Choosing modules with a positive-only power tolerance means the modules will at least perform to their rated specifications under STC, and possibly above. A 200 W module with a +10/-0 power tolerance is warranted to produce between 200 and 220 W. Because of higher output, they also may be appropriate for systems with limited installation space.
Voltage Characteristics. This will vary between modules. Maximum power voltage (module voltage under load and at STC) ranges from 16.5 to 72.3 volts; and open-circuit voltage (module voltage when there is no electrical load and it is exposed to sunlight) ranges from 21.8 volts to 88.1 volts. These values are for STC module cell temperature. Below 25°C (77°F), module voltage increases. Above this temperature, module voltage decreases. This, coupled with the need for the array to stay within the grid-tied inverter’s input voltage range, means that modules need to be selected and configured carefully.