Globally, 1.5 billion people live off the grid, their only access to electricity often limited to
operationally-expensive fossil fuel generators. Solar power has risen as a sustainable and less
costly option, but its generation is variable during the day and non-existent at night. Thanks
to recent technological advances, which have made large-scale electricity storage economically
viable, a combination of solar generation and storage holds the promise of cheaper, greener, and
more reliable off-grid power in the future. Still, it is not yet well-understood how to jointly
determine optimal capacity levels for renewable generation and storage. Our work aims to shed
light on this question by developing a model of strategic capacity investment in both renewable
generation and storage to match demand with supply in off-grid use-cases, while relying on
fossil fuel as backup. Despite the complexity of the underlying model, we are able to extract two
general results. First, we find that solar capacity and storage capacity are strategic complements,
except in cases with very high investment in generation capacity, when they surprisingly turn
into strategic substitutes, with implications for long-term investment decisions. Second, we
develop a simple heuristic to determine which storage technology, within a given portfolio, can
turn a profit in the broadest set of market conditions, and thus is likely to be adopted first. We
find that currently, low-efficiency, cheap technologies such as thermal can more easily turn a
profit in off-grid applications than high-efficiency, expensive ones such as lithium-ion batteries.
We then develop two newsvendor-like approximations of the general model that are analytically
tractable, yield precise values for the optimal investment decisions and profit in some cases, and
provide bounds to the optimal investment decisions and profits in all other cases. To conclude, we
calibrate our models to measure the accuracy of our solutions utilizing real-life data from three
geographically-diverse islands, and then use our approximations to provide high-level insights
on the role that large-scale storage will play in the years ahead as technology improves, carbon
taxes are levied, and solar becomes cheaper.