A method for controlling fossil fuel fired heating systems according to an improved adaptive cyclical control method when operating on backup (auxiliary DC) electrical power. Battery life is prolonged when used in conjunction with such improved method. Operating characteristics of the heating equipment and home or building being heated are monitored and stored in electronic memory during normal periods of operation when grid power is available. These characteristics are used to create control algorithms that are in turn specific to the system, as defined by the heating equipment and conditioned space. When the grid power is lost the control algorithms select the optimal heating mode, determine optimal cycle duration, and estimate the time to exhaustion of the remaining backup power.Operation is divided into sequential modes. When a prolonged low voltage period is detected, the backup power control initiates a short-term preliminary energy conservation mode. As the outage persists, the control transitions to intermediate and long-term energy conservation modes until the backup power is exhausted or grid power is restored.