With power there are essentially two types, the base or "constant" source and the variable source. Gas, coal, hydro, nuclear etc. are considered the constant source (not infinite, but accessible at all times) and wind and solar are variable sources. I have lived off grid for 15 years which would not have been possible without batteries (our constant source) because for some reason the sun doesn't shine 24 hours a day and the wind doesn't blow all the time.
For every cost analysis of renewable energy it MUST take into account the base source, be it batteries, hot salts, gas, coal, nuclear etc. A wind or solar farm may be cheaper to erect than a nuclear plant BUT what is going to be used for a constant source? Solar with batteries (lithium, lead carbon etc) is going for about 50c per KwH for a small system, which is still above even the SA price of 40c/KwH. In all the media about renewables being cheaper and better they all conveniently skip or gloss over the base load or constant source - this is the problem that needs to be solved first and now. Maybe 10 years in the future we will have cheap affective storage, but we do not have it now!