Landlord forced to pay electricity bill according to the bill month

I've shifted to a place on 1st September and Landlord has shared a bill of 5600 approx on 20th November (Bill month - October, November & December) which I've paid by thinking that it's mine. But next day, I've received a bill of 1300 (Bill month - January, February and March). By consulting to Electricity Dept, I've got the confirmation that the bill of 5600 approx was from 21st May to 20th August according to the consumption which should be paid in October, November & December. And the bill of 1300 was from 21st August to 20th November according to the consumption which should be paid in January, February and March. But my Landlord is forcing us to pay by the bill month and not by consumption month due to which I've paid for the electricity that the old tenant has used and not by me. Please advice what can I do.