In 2016 when I attended the Macquarie Bank companies conference in Sydney, I asked the CFO of AGL (one of Oz's largest energy utilities) why their new solar farm would cost A$100/MWh, when at that stage, converting from US dollar costing, it should have been A$20 to A$30 cheaper. His answer, in essence, was that they were just starting down the learning curve: the site was far from the nearest big town, they were struggling to find workers, etc. PPAs (Power Purchase Agreements) aren't published in Oz, unlike the case overseas, so we had only snippets released by AGL and Origin (the two largest electricity and gas utilities) about what renewables were costing. But last year, they were talking about solar at A$75/MWh. According to RenewEconomy, this year at the Energy Australia conference, the talk in the corridors was of A$40/MWh. This is -- obviously -- a dramatic decline, and reflects rising economies of scale, learning curve improvements, and the rapid cost declines in PV cell manufacture.
AGL also stated that coal costs somewhere between A$100 to A$110/MWh. That's without the externalised costs of coal (pollution, global warming) being included in the calculation. When they do their projections, any investor in coal mines or power stations has to consider the very real risk that carbon is going to be priced in the future, as global temperatures rise.
Adding six hours of storage to solar (or wind) would cost A$30/MWh extra to the cost of wind and solar now, and half that in 5 or 6 years' time. Suppose you add 6 hours of storage to new wind or solar PV farms, the average cost of 50% wind plus 50% solar with 6 hours of battery storage would be A$80/MWh now, and A$50-A$60 in 5 years--much less than new coal. Or we could use solar thermal (CSP) which costs A$75/MWh and delivers baseload/dispatchable electricity. Is 6 hours storage enough? Yes, provided there is a blend of solar and wind, and the penetration of renewables in the grid is less than 50 or 60%. The falling costs of storage will allow cost-effective expansion of renewables to 80 or 90% of electricity generated within 10 years. And because of rapid cost declines in the whole spectrum of renewables, that percentage is very likely to be reached by then.
[Update 27/06/18: RenewEconomy reports that AU solar is now down to $50/MWh]
Disclaimer. After nearly 40 years managing money for some of the largest life offices and investment managers in the world, I think I have something to offer. These days I'm retired, and I can't by law give you advice. While I do make mistakes, I try hard to do my analysis thoroughly, and to make sure my data are correct (old habits die hard!) Also, don't ask me why I called it "Volewica". It's too late, now.
BTW, clicking on most charts will produce the original-sized, i.e., bigger version.