The efficiency is typically denoted by the Greek small letter eta: η. In all devices converting the energy, part of the input power is lost and the efficiency is a measure of power loss. This parameter is noteworthy, since the higher the efficiency, the less energy is lost which means that the temperature inside the power supply is lower and as a result, the reliability and service life are increased. Available switched mode power supplies offer efficiencies >90% (transformer or linear power supplies efficiency does not exceed 50%).
η – efficiency (%)
Pout – output power
Pin - input power
Example 1. The efficiency of a power supply with 100 W output power at mains power input of 117.6 W can be calculated as follows:
In the data sheets, the manufacturers usually specify output power and efficiency of the power supply, however, the power input is not usually specified. It can be easily calculated using the following equation.
Example 2. Power supply with 150W output power and 86% efficiency. Mains power input can be calculated as follows:
Power loss as a thermal energy (Pd – power loss) can be calculated using a simple equation (take away the generated power from the power input).
In this case, 24.4 W is lost as a thermal energy at full load. Those 24.4 W increase the temperature inside the enclosure and the temperature of internal components.