In a Gas Sales Agreement (GSA) if the buyer takes more than his TOP or ACQ amount of gas in any year (depending on what was agreed), he will then receive a credit for the overtake. This is aggregated with all overtake credits from previous years as a carry forward balance.

Thereafter, if the buyer takes less than the ToP amount, he can then reduce his ToP liability by the amount of the carry forward balance. The buyer will want to negotiate as long a period as possible and certainly longer than 1 year. If the ToP is high, then the carry forward period should be longer as there is smaller chance that the buyer will be able to enjoy the carry forward as he will be meeting his ToP obligations more frequently