In theory, the power that provides power supply can be as great as the power of the machine burns. In reality, this represents an efficiency of 100%, a level of performance that provides electricity can never achieve. There is always some wastage when 110 V or 220 VA / C power is converted in to various D / C voltage levels in power supply has some waste, with the majority of energy waste be felt as heat produced in the case of PC This means that the nominal power supply that can deliver must be strictly less than the energy it consumes before starting the process of transforming power.

By calculating the ratio between energy production and energy consumed, we produce a number between zero and one. For example, the net energy output of 450 W divided by gross consumption of 550 W at full load produces a value of 0.818. This is the number that tells us the efficiency of the power supply. Usually the efficiency index is known as a percentage value, which may be calculated by multiplying the previous ratio by 100 to produce in our example, a value of 76.4%.

Rated power seller power supplies are not always the maximum power that the device can offer. A 350 W PSU with an efficiency rating of 70% must therefore consume a maximum of 500 W, although this occurs only when the components that feed readers actually consume around 350 watts. The true effectiveness of a power supply is not a constant value or, rather, it changes with the amount of energy that the device delivers at any given time. The ATX12V Power Supply Design Guide requires that USP offers a minimum efficiency of 65% under light load, 72% under normal load, and 70% at peak load. There is also an efficiency regime recommended that these high levels to 75% for light loads, 80% for normal loads, and 77% at peak load. Here, the word "charge" must be understood as the energy system, measured in amperes.