How is NOCT defined?
Normal Operating Cell Temperature (NOCT) is a testing standard geared to the operational conditions of solar cells, defined as the temperature reached by open circuited cells in a module assuming 800W/ m² irradiance, 20°C ambient temperature and wind speed of 1m/ s with the PV module at a tilt angle of 45° and its back side open to the breeze (as opposed to conditions where panels are mounted on roofs and heat builds up under the panel).
Similar to PTC, NOCT conditions are an approach to mirror real-world conditions. It is applied to calculate real available wattage on an average day and is a comparably more strict parameter that is required by a range of energy rating and output performance standards.
It is the major testing condition alternative to Standard Testing Conditions (STC) and also used for solar panel performance quality testing during mass production.
Impacts of module design and mounting on NOCT ratings
Materials used for modules as well as packing density may have major impacts on NOCT and result in distorted ratings. For example a rear surface with a lower packing density and thus reduced thermal resistance can result in temperature differences of over 5°C.
Both conductive and convective heat transfer are significantly affected by the mounting conditions of the PV module. A rear surface which cannot exchange heat with the ambient (i.e., a covered rear surface such as that directly mounted on a roof with no air gap), will effectively have an infinite rear thermal resistance.
Similarly, convection in these conditions is limited to the convection from the front of the module. Roof integrated mounting thus causes higher operating temperature, often increasing the temperature of the modules by 10°C.