Rated Power at STC (Watts)

Definition: Module power rating at STC—1,000 watts per square meter of solar irradiance at 25°C (77°F) cell temperature.

Importance: Because module power output depends on environmental conditions, such as irradiance and temperature, each module is tested at STC so that modules can be compared and rated on a level playing field. When less sunshine hits the module, less power is produced. Likewise, the hotter it gets, the less power your modules will produce.

STC references cell temperature—not ambient air temperature. As dark PV cells absorb radiant energy, their temperature increases and will be significantly higher than the ambient air temperature. For example, at an ambient air temperature of about 23°F, a PV cell's temperature will measure about 77°F—the temperature at which its power is rated. If the ambient air temperature is 77°F (and irradiance is about 1,000 W/m2), module cell temperature will be about 131°F and power output will be reduced by about 15%. Other factors, like rated power tolerance (discussed below), can impact module power production as well.

Rated Power Tolerance (%)

Definition: The specified range within which a module will either overperform or underperform its rated power at STC.

Importance: Power tolerance is a much-debated module specification. Depending on the module, this specification can vary greatly—from as much as +10% to -9%. A 100 W module with a -9% power tolerance rating may only produce 91 W straight out of the box. With potential losses from high temperatures, it will likely produce even less than that.

Because modules are often rated in small increments, it is not uncommon for modules that fall under the lower power tolerance of the next model to be rated as a higher wattage module. Case in point: A module with a +/-5% power tolerance rating that produces 181 W during the factory testing process could be classified as a 190 W module, as opposed to a 180 W module. For maximum production, look for modules with a small negative (or positive only) power tolerance.

Rated Power Per Square Foot (Watts)

Definition: Power output at STC, per square foot of module (not cell) area. This is calculated by dividing module rated power by the module's area in square feet. Also known as "power density."

Importance: The higher the power density, the less space that is needed to produce a certain amount of energy. With some of the newer-generation modules, power density values are higher due to increased module efficiency. The greatest

New technologies, like these Sanyo HIT modules, blur the divisions between traditional silicon types and gain higher efficiencies.

variation in this specification is in comparing crystalline PV modules to thin-film modules. If space is tight for array placement, consider choosing modules with higher power densities, though more efficient modules can be more expensive. Choose modules with lower power densities, and you'll need more modules for the same amount of energy. That means more infrastructure (module mounts, hardware, etc.) and more installation time. (See "Solar-Electric Options— Crystalline vs. Thin-Film" in HP127 for more information.)

Module Efficiency (%)

Definition: The ratio of output power to input power, or how efficiently a PV module uses the photons in sunlight to generate DC electricity.

Importance: If 1,000 W of sunlight hit 1 square meter of solar module and that solar module produces 100 W of power from that square meter, then it has an efficiency of 10%. Similar to power density, the higher the efficiency value, the more electricity generated in a given space.

Commercial PV Trend

While many new PV companies are manufacturing ULlisted modules, several will only sell directly to large commercial/industrial and power-plant PV projects. You might find these modules listed in inverter string-sizing programs and on the Go Solar (California Energy Commission) Web site from companies such as First Solar, Siliken, and Solon, but you will not be able to buy them from PV distributors or dealers.

The trend toward commercial-scale-only sales is particularly evident with thin-film producers. Although consumers may hear of rapid growth in thin-film technology and market share, many thin-film options are not available for residential applications. This guide excludes modules (crystalline or thin-film) that are only available to the commercial or industrial PV market.

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