The ordinary or commercial efficiency of a transformer is defined as the ratio of output power to the input power.
Commercial efficiency = Output power ∕ Input power
Importance of All Day Efficiency
There are certain types of transformers whose performance cannot be judged by commercial efficiency.
For instance, distribution transformers used for supplying lighting loads have their primaries energized all the 24 hours in a day but the secondaries supply little or no load during the major portion of the day.
It means that a constant loss (i.e., iron loss) occurs during the whole day but copper loss occurs only when the transformer is loaded and would depend upon the magnitude of load.
Consequently, the copper loss varies considerably during the day and the commercial efficiency of such transformers will vary from a low value (or even zero) to a high value when the load is high.
The performance of such transformers is judged on the basis of energy consumption during the whole day (i.e., 24 hours). This is known as all day efficiency or energy efficiency.
What is All day Efficiency?
The ratio of output in kWh to the input in kWh of a transformer over a 24-hour period is known as all day efficiency.
ηall-day = kWh output in 24 hours ∕ kWh input in 24 hours
In the design of such transformers, efforts should be made to reduce the iron losses which continuously occur during the whole day.
Note. The efficiency of a transformer means commercial efficiency unless stated otherwise.