This is the 1127th most frequent Russian word.
электроэнергии
Electric energy.
The word "электроэнергии" is used in the genitive case to indicate the concept of "electricity" in the matters of conservation or usage.
В нашем городе внедряют новые технологии для экономии электроэнергии.
New technologies are being implemented in our city to save electricity.
"Электроэнергию" here is in the accusative case, referring to "electricity" in the context of something consumed and billed for.
Счёт за электроэнергию снова вырос в этом месяце.
The electricity bill increased again this month.
"Электроэнергии" is in the genitive case, describing "electricity" in the context of its generation.