Utilities Charge After Move Out

My question involves landlord-tenant law in the State of: Georgia
I moved out from the property a month ago (this July). Never received any thing. Used Georgia Power under the landlord's name (we didn't open an account). We paid 40 dollar each month for the "utility package". Now the landlord sent us a letter, stating that we exceeded the usage from last December to this July. and each of us (4 people) need to pay the landlord 40 dollars.
Is it legal for them to charge us extra utility after we moved out? Thank you for your answer!

Tell us what your lease agreement says about the subject.

For example, if you agreed to pay the entire bill and you paid $40/month toward an estimate pending receipt of an updated figure, then you owe the difference.

Sounds to me like your LL was using one of GP's special programs. If I recall correctly how it works, the monthly bill is the average of the previous annual billing. The catch, which many people overlook/don't understand, is that if the power consumption for the new period exceeds that of the previous one, the customer is responsible for any overages. In this case, whether you will owe or not depends on the language of the agreement as Mr.Knowitall has pointed out.

The $40 the LL wants...is that a flat $40 or $40 per month?

Thank you Mr. Knowitall!
My lease agreement says if we choose to have a "power package" (which we chose to have), then we pay $40 each month. If we use more than the cap each month, the extra will be deemed as rent of the next month. (I just saw it after I wrote the post)
So the question now is since they said the exceeded amount would be deemed as the rent of the next month, and they never noticed us during the period of the lease, instead they sent us a mail a month after we moved out, are we still liable for the extra electricity charge?
Thank you very much for the answer!

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free9man, Yes now I think you are right about the how the electricity package works. The $40 dollars the LL wants now is a flat $40 dollars from every one and that's it I think. But as it is stated in the agreement, the exceed amount will be deemed as the rent of the next month. The LL never informed us during our stay.
Thank you very much for your answer!:)