Realizing highly available datacenter power infrastructure is an extremely expensive proposition with costs more than doubling as we move from three 9s (Tier-1) to six 9s (Tier-4) of availability. Existing approaches only consider the cost/availability trade-off for a restricted set of power infrastructure configurations, relying mainly on component redundancy. A number of additional knobs such as centralized vs. distributed component placement and power-feed interconnect topology also exist, whose impact has only been studied in limited forms. In this paper, we develop detailed datacenter availability models using Continuous-time Markov Chains and Reliability Block Diagrams to quantify the cost-availability trade-off offered by these power infrastructure knobs.