This is the 658th most frequent Ukrainian word.
електроенергії
Electricity.
Here, 'електроенергію' is in the accusative case and is the direct object of the action to conserve.
Зберігайте електроенергію, вимикаючи непотрібні прилади.
Save electricity by turning off unnecessary devices.
In this context, 'електроенергії' again appears in the genitive case, indicating possession or relationship regarding an issue concerning its amount.
Проблема недостатньої електроенергії в регіоні повинна бути вирішена.
The problem of insufficient electricity in the region must be addressed.
In this sentence, 'електроенергії' is used in the genitive case to indicate a large quantity being consumed.