Power rating

In electrical engineering, mechanical engineering and other disciplines, the power rating of equipment is defined as the highest power input allowed to flow through particular equipment. According to the particular discipline, the term "power" may refer to the electrical or mechanical power. A power rating can also involve average and maximum power, which may vary depending on the kind of equipment and its application.

Power rating limits are usually set as a guideline by the manufacturers, protecting the equipment and simplifying the design of larger systems, by providing a level of operation under which the equipment will not be damaged while allowing for a certain safety margin.

Equipment types

Dissipative equipment

In equipment which primarily dissipate electric power or convert it into mechanical power, such as resistors, and speakers, the power rating given is usually the maximum power that can be safely dissipated by the equipment. The usual reason for this limit is heat, although in certain electromechanical devices, particularly speakers, it is to prevent mechanical damage. When heat is the limiting factor, the power rating is easily calculated. First, the amount of heat that can be safely dissipated by the device, , must be calculated. This is related to the maximum safe operating temperature, the ambient temperature or temperature range in which the device will be operated, and the method of cooling. If is the maximum safe operating temperature of the device, is the ambient temperature, and is the total thermal resistance between the device and ambient, then the maximum heat dissipation is given by

Nominal power (photovoltaic)

The nominal power is the nameplate capacity of photovoltaic (PV) devices, such as solar cells, panels and systems, and is determined by measuring the electric current and voltage in a circuit, while varying the resistance under precisely defined conditions. These Standard Test Conditions (STC) are specified in standards such as IEC 61215, IEC 61646 and UL 1703; specifically the light intensity is 1000 W/m², with a spectrum similar to sunlight hitting the earth's surface at latitude 35°N in the summer (airmass 1.5), the temperature of the cells being 25 °C. The power is measured while varying the resistive load on the module between an open and closed circuit (between maximum and minimum resistance). The highest power thus measured is the 'nominal' power of the module in watts. This nominal power divided by the light power that falls on a given area of a photovoltaic device (area × 1000 W/m²) defines its efficiency, the ratio of the device's electrical output to the incident energy.