Apart from general efficiency of transformer, it is important to note all day efficiency of transformers on a regular basis. To find this all day efficiency, a ratio of energy delivered within a 24 hour period is to be placed against energy that is placed within that time period.
Since these transformers are specifically used in lighting of residence and therefore, it is the 24 hour period that is to be taken into consideration. However, this circuit has to be connected in a fixed line, and therefore chances of core loss being consistently supplied are taken into consideration. So it has to be seen that total loss amount should be minimal.
In case of copper losses, it has to be seen that transformer distribution is designed in a manner that only at lighter loads; they have power efficiency that is maximum. Power transformers that are available are loaded with full load copper loss that has about two times of the total loss amounts. So it is important to note the level of variation that happens on an hourly basis, to get a complete idea of the efficiency of that transformer.
So, if you have difficult sums, you can surely use this formula and details for solving them.