Definition: The all day efficiency is defined as the ratio of output in kilo watt hour (KWh) or watt hour (Wh) to the input in kWh or Wh of a transformer over 24 hours.
Simply the ordinary or commercial efficiency of a transformer is defined as the ratio of the output power to the input power.
Some transformer efficiency cannot be judged by simple commercial efficiency as the load on certain transformer fluctuate throughout the day. For example, the distribution transformers are energized for 24 hours, but they deliver very light loads for the major portion of the day, and they does not supply rated or full load and most of the time the distribution transformer has 50 to 75% load on it.
As we know, there are various losses in the transformer such as iron and copper loss. The iron loss takes place in the core of the transformer. Thus, the iron or core loss occurs for the whole day in the distribution transformer.The second type of loss known as copper loss takes place in the windings of the transformer also known as variable loss. It occurs only when the transformers are in the loaded condition.
Hence, the performance of such transformers cannot be judged by the commercial or ordinary efficiency, but the efficiency is calculated or judged by All Day Efficiency also known as operational efficiency or energy efficiency which is computed on the basis of energy consumed during a period of 24 hours.
To find this all day efficiency, firstly you have to know the load cycle of the transformer.