A power industry always looks for an optimal generation mix. This paper investigates the potentiality of a wind turbine generator (WTG) and a photovoltaic generator (PVG) as competing candidates in a generation mix of an isolated area. The investigation considers two basic aspects of generation expansion analysis: reliability and cost. The WTG, at its present status related to cost and also at its possible future status, is compared with the conventional thermal unit. Similar investigation is made in the paper for the PVG. Unlike many attempts in the past, the temperature dependent probabilistic models of WTGs and PVGs are developed for the evaluation of reliability and cost The paper also shows an application of temperature dependent probabilistic models of WTGs and PVGs in generation expansion analysis.