Abstract The traditional outage models with constant failure rates do not truly reflect the environmental and operating conditions nor the effect of repair operations on power systems. This paper proposes two useful approximations for the availability function based only on the first three of the lifetime distribution. We compare the availability function of a power transformer using the numerical inverse of the transformed availability function. It also suggests methods to verify the effectiveness of the approaches. Finally, it outlines the proposed approximations to validate the efficacy of the approximations.