When two modules of the same dimensions are exposed to the same sunlight intensity, the more-efficient one will yield more watts per square foot. Thus, module efficiency and rated power per square foot are closely related.
Watts per square foot is easy to calculate—divide the module’s rated power by its dimensions in square feet. For example, a 250 W module that measures 40 inches by 65 inches (18.06 ft.2, 40 in. × 65 in. ÷ 144 in.2/ft.2) produces 13.84 W per square foot (250 W ÷ 18.06 ft.2).
Efficiency is calculated based on the nameplate power rating of a module, which what it produces when subject to 1,000 W per m2, called “full sun.” Calculating the module size in square meters means a ratio can be made to calculate efficiency (1 ft.2 = 0.0929 m2). The 250 W module that makes 13.84 W per ft.2 is just a little less than 15% efficient:
18.06 ft.2 × 0.0929 = 1.678 m2
250 W ÷ 1.678 m2 = 149 W per m2
149 W per m2 ÷ 1,000 W per m2 = 0.149 = 14.9% efficient
More than 530 of the modules on our list are at least 14% efficient; the field narrows to about 140 modules that boast at least a 15% efficiency rating; only 20 modules have an efficiency of 16% or more.
No matter how you slice it, SunPower modules are tops in both efficiency and watts per square foot. The company’s monocrystalline silicon modules feature high-efficiency cells and use back contacts, eliminating the traces and conductors between cells that appear on the front of most modules and which block photons from sunlight. This translates to more surface area, more current, and more power per square foot.
Using efficient modules can be particularly effective when space is limited. However, if it costs too much, the bit of extra efficiency may not be worth it. Selecting modules based on cost—“dollars per watt”—frequently trumps efficiency when there is enough space for the array.
Unfortunately, the saying, “you get what you pay for” doesn’t necessarily hold true when you’re paying dollars per watt for PV modules. That’s why you need to pay attention to power tolerance, the percentage that the actual power output of a module can vary from its STC rating.
For example, a 200 W module with a +/-5% power tolerance may produce anywhere from 190 to 210 W at STC. Product lines that are often differentiated in 10 W increments become blurred when the 210 W version of the same module may test between 200 W and 220 W. About 400 modules on our list have the most common power tolerance of +/-3%.
More than 350 of the modules listed have positive-only power tolerance, which means that the module will produce at least its rated power at STC, if not a little bit more. Nearly 75 on the list have a power tolerance of +5/-0% or better, including modules by Kyocera, Perlight Solar, Schüco, Solon, Suntech, Sanyo (+10/-0%), and Schueten Solar.