However, when you aggregate the demand from millions of households, it becomes reasonably predictable from day to day and month to month. The chart below shows Californian electricity demand on a hot day in 1999 (it doesn't show net demand, that is, demand net of power generated by solar panels, which produces the "peaking duck" curve)
There are still fluctuations of course, with daytime demand peaking at twice the level that demand falls to at its low point in the wee hours of the morning, but the curves are much smoother. On hot days, conveniently, the demand peak partially coincides with a likely supply peak from solar. With a few hours of storage, solar could provide for all daytime demand, at least for places between latitudes 35 or 40 N and S.
Now the key point here is that if the grid had to be built to supply the peak power demands of each individual household, electricity would be extremely expensive. That's why "going off the grid" is too expensive to do, no matter how tempted we are by the outrageous electricity prices and network fees charged by our utilities. An individual house or small business would need far too much capacity to make sure it never ran out of power. For the grid as a whole, far less spare capacity is needed, because individual peaks and troughs are averaged out. This spare capacity is often provided by gas peaking plants, which can start up quickly and turn off quickly whereas most "baseload" power (coal and nuclear) can't be easily scaled up or down.
The same averaging process is true when you are looking at individual sources of supply too, including the variable supply from renewable generating sources. Take a look at the chart below (from this website) It shows the wind output of various Australian wind farms expressed as a percentage of total capacity. The black line shows total Australian wind farm output, also as a percentage of total capacity. See how individual wind farms can fluctuate from nearly 100% of capacity down to 0% in the space of a few hours, but how the total of all wind farms across the country (the black line) varies between 30% and 50%. On average, over time, Oz wind farms produce 30 to 35% of nameplate capacity.
This is all relevant because when we're working out how much backup or storage we need, there are some who maintain that each wind or solar farm needs 100% backup from a conventional fossil fuel generator. And of course, this makes wind and solar appear impossibly expensive (that's the point, I suspect). But we don't actually need that much back up or storage. Here're the views of the CSIRO (Commonwealth Scientific and Industrial Research Organisation), an Australian independent government-funded research body:
CSIRO Energy chief economist Paul Graham says that additional storage is not needed for up to 40 to 50 per cent wind and solar penetration. That’s because the grid can rely on existing back-up ( built to meet peaks in demand and for when coal and gas “baseload” generators trip or need to be repaired).
Beyond those levels, storage needs to be part of the equation. But again, not as much as many would think. But as the back-up generators gradually exit the grid, they can be replaced by various storage types, until storage then becomes the principal form of back-up and grid security on the grid.The CSIRO modelling showed that at very high levels of wind and solar, a maximum of half a day’s average demand was needed for storage. In some areas of the grid, only around three hours might be needed.
This is an important point, because some renewable critics say that about a week’s worth of storage is needed, and multiples of wind and solar capacity required for back up. These would be the same people that argue that climate science is a hoax, but it is a view that has more traction than it should.
Graham says the CSIRO modelling indicated that at those very high levels, about 0.8GW of back-up was required for about every GW of wind and solar capacity. This is around the same amount of back up capacity currently needed by centralised power plants to meet peak demand and outages.
[Read more here; my emphasis]
What's true for Australia is likely true for most other geographies where power is generated from a variety of sources, except as I discuss here, countries in high latitudes where the majority of renewable power will come from wind. Though note that the CSIRO's forecasts assume that more than half of Australia's power will be wind generated by 2050.
Lazard have recently updated their LCOEs for different generation sources, and for the first time have provided an estimate of solar with 10 hours of storage. They come up with a cost of US$92/MWh. Average coal (in the US, ignoring the implicit cost of pollution, i.e., without a carbon tax) is $100/MWh. Combined cycle natural gas is cheaper, averaging US$63/MWh, but as the chart below shows, natural gas prices fluctuate widely. And if--when--fracking is curtailed because of its environmental costs, the price is likely to rise further. So in future gas may not be cheaper than solar plus batteries, even without a carbon tax.
You might notice that though output from an generator fuelled by fossil fuels is stable, apart from outages, its costs are not stable; while on the other hand output from renewable energy sources is variable, but its costs are fixed, because its "fuel" is free. A sort of Heisenberg uncertainty principle for electricity generation. But utilities and users value price stability as much as they value supply reliability, Even if gas is cheaper, they might still find the price and supply stability promised by renewables plus storage as very attractive.
Lazard has made a much more detailed costing of storage than I was able to. What it shows is that for most of the world, even when you add in the necessary storage, solar can provide us with all our electricity needs. And when I say most of the world, I mean India, China, Africa, the southern US and Europe, Australia and South America., as you can see from the chart below. These regions can all be powered from solar plus storage. Right now. As cheaply as or more cheaply than coal. And with more stable costings than gas. Even without a carbon tax. These locations might also have wind energy, and that's a bonus, because the output from wind plus solar is on average less variable than the output of either alone. But they can do it on solar. Right now.
Global temperatures are rising by 0.2 deg C per decade on average. If it takes us 2 decades to switch our entire generation fleet to renewables, and to electrify most transport, temperatures will still have risen by another 0.4 deg C. The 1 deg C rise we've already had has caused enough problems. We can't afford to wait, and now we have no reason to either. Switching to renewables is now affordable. And it won't cost us the earth.