Other definitions of payback time are the time required for the annual solar savings to become positive and the time required for the cumulative solar sav ings to become 0. These can be determined by life cycle analysis. Using the results of Example 12.7, the payback time is
1. The time required for the annual solar savings to become positive = 4 years
2. The time required for the cumulative solar savings to become zero = 15 years