In various industries, system availability assessment has been extensively investigated. Particularly, a stable power supply system is essential and critical in many service systems such as communication networks, mass transit systems, data storage centers and product manufacturing processes, etc. When an unreliable power supply unit fails, the repair services may be unavailable and a random period of (retrial) time for contact and confirmation is needed. Thus, this paper compares four retrial power supply systems with general repair time, warm standby units, and imperfect coverage possibility. That is, the power generation equipment is assumed unreliable and may fail anytime. The steady-state probabilities and the associated availabilities of four retrial systems are derived explicitly. A comparative analysis of the availability and the cost-benefit ratio among four retrial systems is presented. The efficiencies of four considered retrial systems are sorted and the optimal retrial system is provided. Based on the calculated numerical results, the management insights for practical applications are also proposed for decision making.