It's hot so I've had the air conditioner on day and night. The electric bill will be scary.
「 エアコンをフル稼働させているので電気代が心配。」は英語で「The air conditioner is on day and night (at full strength/at full capacity) so I'm worried about the electric bill.」という表現が良いと思います。
フル稼働ーFull strength・Full capacity
電気代ーElectric (utility) bill
アメリカ合衆国 
