Why this word is plural? Is singular form incorrect?

Energy bill - one bill
Energy bills - more than one bill

But what about electric bills?

My answer remains the same.

Because of many appliances?

No. My answer is that both forms are possible.

If you provide an example sentence, I can tell you whether the noun should be singular or plural.

If you install cavity wall insulation, then your electric bills will be less. Is it a correct one?

faisalkhanIf you install cavity wall insulation, then your electric bills will be less. Is it a correct one?

Yes. The installation would affect not only your next electric bill but also subsequent ones; therefore, the plural form is the right choice.

Here the singular form is used:

My electric bill was gargantuan last month.

faisalkhanIf you install cavity wall insulation, then your electric bills will be less. Is it a correct one?

People often use the singular there: "your electric bill will be less." They mean each bill, month by month.