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.