[Hat tip to Climate Denial Crock of the Week for the clever title.]
Here's a short video from Vox which explains the difficulties the rising penetration of solar is creating for the managers of the grid.
Before the widespread adoption of rooftop solar, the (Californian) electricity demand curve looked like this:
There was a morning ramp up and then another ramp up in demand from 6 pm to 10 pm.
But as rooftop solar spread, the daytime net demand (the demand visible to the grid operator CAISO) fell each year and is forecast to continue to fall.
This produces a curve which looks like a duck, hence the "duck curve" or "peaking duck curve". As you can see, from the point of view of the grid operator there is a huge surge in demand in the evening, made up of an actual increase in demand and a tail-off of rooftop solar generation. This requires that peaking gas plants need to be started up to cater for the demand. Peaking gas is expensive, because you need to pay for the plant maintenance, depreciation and debt repayment from just a few hours of use each day.
It produces a second consequence too: overgeneration.
Because baseload power generators (coal and nuclear--the tan line in the chart above) can't really be scaled up and down or switched off over midday when solar supply peaks causing net demand to drop below supply from baseload generators, output has to be curtailed. I.e., any generator which can be switched off will be asked to. Otherwise the grid would burn out.
The solution to those dual problems is of course storage. If storage is cheap enough then it would pay utility-scale solar farms to install storage. When the grid operator requests that output into the grid be curtailed, the solar farm would just switch its output from the grid to its batteries. And when net demand ramps up in the evening, the solar farm would simply supply it from the power it stored earlier in the day. By the way, this applies to wind too, even though wind farm output doesn't fall off at night, because the grid operator will ask any dispatchable power supplier to shut down when supply is excessive, even when it's not its fault.
So how much storage would be needed? Well, from the POV of the grid operator or any individual wind or solar farm, any storage would be better than nothing if it's cheap enough. A renewable generator would reduce curtailment and the grid would reduce the need to ramp up expensive gas peaking power plants. But to remove or significantly flatten the peak of the duck curve plus remove the need for curtailment would require 4 to 6 hours of storage.
And how cheap is cheap enough? 1 hour of battery storage costs about $4.5/MWh, 6 hours about $27. The cost of wind is $30-$60/MWh, the cost of solar $43-$53/MWh. But it depends on the wholesale price, which fluctuates during the day. The cost of peaking gas is $156-$210/MWh, and of course the need to use peaking gas would drive up the wholesale price in the evening demand peak. So a solar or wind farm installing battery storage would avoid curtailment (=zero income) at midday while earning (potentially) $150/MWh plus in the evening.
The big battery in South Australia has shown what is possible, and already wind farms in that state (60% renewables penetration) are retrofitting battery storage to avoid the costs of curtailment and to take advantage of evening peak wholesale prices. As battery costs fall--and they should halve over the next 3 years--this will become widespread wherever the penetration of renewables rises enough to either require curtailment or to affect wholesale prices. Also, as batteries become cheaper, households and businesses will install behind-the-meter storage, thus eliminating the daytime net demand decline.
The duck curve, curtailment and the evening ramp up would be a major problem in the absence of cheap storage. But with cheap storage, batteries will soak up daytime sunshine so avoiding curtailment while reducing the need for peaking gas in the evening.
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.