Mmm ... I'm really at a loss on this subject
From a technical (engineering) perspective it all comes down to two things really. Energy and power - and they're very different things.
Energy is the total. It's like saying how far you drove. Note that just because you've traveled 1000 km, that says absolutely nothing about how you did it or how long it took, it's only a measurement of how far you've gone.
Power is the rate. It's like saying the speed at which you traveled. If you're traveling at 100 km/h then that tells you the rate but it doesn't tell you how far you've gone. Could be on a suburban highway or you could be driving between Perth and Adelaide.
So energy = how much. Power = rate of use.
Now the great problem with electricity is that it's like light or sound, you can't store it as such. You can store a means of producing sound yes but you can't store actual sound anywhere and for practical purposes it's the same with electricity, we can store water in a dam or coal in a fuel hopper but we can't in practice store the electricity (even a battery is a chemical change).
So production needs to match consumption in real time and that's where all this gets hard. In places like SA, where maximum rate of consumption is about 4 times the minimum rate, it's especially difficult and the great problem there is that wind and solar generation, driven by the weather, don't always align well with when the electricity is actually needed.
Looking at Qld, NSW, Vic, ACT, Tas, SA (the National Electricity Market states) as a whole, at 12:00 today (eastern states time) when combined solar output peaked it was supplying 12.9% of the total demand of 26,728 MW. So that's 4005 MW of solar and 22,723 MW of everything else.
6.5 hours later at 18:30 demand peaked at 30,137 MW and solar was zero.
So there's the great difficulty in using solar energy. We get the maximum supply at a time when demand is a fair way below maximum, then we get no supply when we need it most.
Wind doesn't have the same daily cyclic pattern as solar but it does still vary very significantly and often dies out at just the wrong time. Eg yesterday at 14:30 demand was 23,100 MW which was the low point for the afternoon. Of that solar was 3051 MW and wind was 2783 MW. Then just 4 hours later demand has risen to 28,274 MW but wind has dropped to 1892 MW and solar's gone to zero.
Those figures are total across the NEM states but at a local level far more extreme things can and do happen. Eg available output from wind farms can exceed 100% of load in SA on occasion, that has actually happened it's not just some theoretical calculation. At other times, and it has also actually happened many times, every wind farm in SA simultaneously has zero or negligible output.
Now in the context of all that I must point out that shutting down and restarting coal-fired plant doesn't come cheap, there's a lot of cost there both economic and emissions and they also have some pretty serious limits in how low the output can be taken whilst continuing to run. It varies but in general they can't really go below 30% - 55% of capacity depending on the plant in question. Now if you get a lot wind and sun on a mild afternoon (not much heating or cooling being used so low demand for electricity) then that situation suddenly becomes a very real problem. Lots of wind and solar, not much load, coal plants don't want to shut down given that starting up again takes hours and they'll need to be running flat out at 6 o'clock that night. It's manageable in a technical sense, just run coal at minimum and cut the output from wind and solar, but that's rather wasteful economically and not a great outcome environmentally either.
Build enough large scale storage, be that hydro or something else, and all these problems can be overcome. That takes considerable time though - hydro projects tend to be measured in years not weeks or months, and big $. That said, once built it's all incredibly durable stuff.
Batteries are an option for storage but in short they're the quicker but more expensive and shorter lived way to go about it all. There's a role to a point but for the big "grunt work" type of application they're an expensive choice.
I'm very sure it's all solvable though. Just needs the politicians to make their mind up over things like emissions targets and then get out of the way and stop playing politics with it all.
An important point is that nothing actually needs to be invented here. The required technology is all proven stuff and it's just a case of applying established electrical, civil, materials etc engineering to work out the details then it's civil construction, trades etc to build it. There's no "trying to put a man on Mars" aspect to it all.