A study of the growth dynamics of the installed capacity of the Nigerian Power System from 1963 to date was done. Perspective available generating capacity was estimated based on service life of existing installed capacity. The sufficiency of generating capacity for future normal operating modes of the system was estimated based on a power load growth forecast from 2003 to 2020. An analysis of the results shows that all the installed capacity since 1963 to the late 80s would have expired by 2020 i.e outlived their rated service lives. At present, only about 4.9 GW (80%) of total installed capacity is valid. Considering the national goal of significant economic development and estimated load forecast of over 8 GW by 2020, there is an urgent need to renovate or replace expired generating units besides the planned building of new power stations which have long gestation periods.