Penetration of renewable energy generators into energy systems is increasing, but the intermittency of these generators makes supplying energy reliably and cost effectively difficult. Storage technologies are proposed as a means to increase the penetration of renewable energy, to minimize the amount of curtailed renewable energy, and to minimize required back up supply. This paper investigates storage sizing methods which utilize economic optimizations and statistical or probabilistic calculations. The results show that probabilistic methods are not sensitive to cost and yield reliability-based solutions. However, the economic optimizations consistently yield the most cost effective solutions. Under excellent renewable energy conditions the probabilistic solutions are nearly as cost effective as economic solutions but offer improved system reliability.