We study the economic viability and optimal sizing and siting of a hybrid plant that combines a ringwall hydro
storage system with wind and solar power plants (ringwall storage hybrid power plant, RSHPP). A real options model
is introduced to analyze the economics of an onshore RSHPP, an in particular of the varying storage volume in light
of the stochastic character of wind and solar power, as well as the optimal investment timing under uncertainty. In
fact, many uncertainties arise in such a project. Energy production is determined by the stochastic character of wind
and solar power, and affects the optimal size of the storage device. Monte Carlo simulation is performed to analyze
the following sources of uncertainty: (i) wind intensity and solar irradiation; (ii) future electricity price; and (iii)
investment costs. The results yield the optimal size of the storage device; the energy market on which the operator
should sell the electricity generated; numerical examples for two different RSHPP application scenarios; and a real
options model for analyzing the opportunity to defer the project investment and thus to exploit the value of waiting.