Following discussion about this in the Electric Vehicles thread, here's some facts and figures regarding power supply.
I'm using Victoria as the example since it's in the worst position with electricity supply adequacy at present. That doesn't guarantee that some other state won't have a problem whilst Victoria avoids one, there's an element of luck involved, but the numbers are worse in Vic and thus it is the harshest test when it comes to being able to charge electric cars without overloading supply.
As a brief summary - power supply is already extremely stretched during heatwave conditions, load shedding is indeed a very real prospect, but even on those days that issue exists only for a few hours. Charging EV's overnight or even during much of the business day simply isn't a major concern. It's late afternoon - early evening that is the problem as the sun sets (solar generation falls) and the use of air-conditioning peaks as well as people arriving home and using electricity for various purposes etc.
Following is a purely technical (electrical) analysis for those interested. You could deduce some financial relevance from it possibly that that isn't the focus. Figures are all MW not $ from here on.
First, some basic data:
Total coal, gas and hydro plant installed in Victoria (there's no diesels in Vic) = 9565 MW (including Murray 1 & Murray 2 which are on the border with NSW and part of the Snowy scheme and including the full output of Hume power station which can be sent to either Vic or NSW as required).
Total discharge power of large scale batteries in Victoria = 75 MW
Transmission limit SA to Victoria = 700 MW
Transmission limit Tasmania to Victoria = 478 MW
Transmission limit NSW to Victoria = a complex answer since the lines are shared with generators in Victoria such that it's largely a "one or the other" scenario. Realistically though, with all that generation running an additional ~200 MW can be pushed through from NSW. In addition to supply from NSW is effectively a sort-of backup to that generating plant (all of which is hydro) since if that fails then without it running, more can be pushed through from NSW assuming there's capacity available.
Wind generation in Vic peaks at just under 1900 MW and large scale solar tops out at about 285 MW.
Total generation in Vic + batteries + supply from interstate = 13,200 MW.
Historic maximum demand = 10,490 MW (all time) and 9618 MW (past 12 months).
OK then, so 13,200 MW supply versus 10,490 MW maximum demand so what's all the fuss about then? This doesn't look to be a problem?
Unfortunately all is not as it may at first seem. Going through the issues:
First problem is that demand peaks during hot weather and usually does so late in the day.
*We can forget any major contribution from solar at this time. It won't be zero but it'll be well on the way down for a late afternoon or early morning peak. Only if, due to unusual weather (which has happened in the past so it's plausible albeit not normal) does demand peak during the middle of the day.
*The big one - heat directly cuts the output of generating plant. It's to the point that there are official "winter" and "summer' ratings for that reason. It doesn't affect hydro plant but it isn't at all kind to gas turbines.
Capacity of coal, gas and hydro plant during mild weather = 9565 MW
Capacity of coal, gas and hydro plant under 41 degree conditions = 8947 MW
(Note that AEMO uses 41 degrees as the reference for hot conditions in Victoria, other states it's different temperatures).
Second problem is that hot weather in Victoria tends to not be isolated to Victoria and usually affects SA at the same time.
*So that 700 MW from SA is by no means assured, it literally depends on either it not being hot in SA at the same time or there being substantial output from SA wind farms. Both have occurred in the past, and both have failed to occur in the past. It's hit and miss.
Third problem is that the chance of all generating plant actually working perfectly is really quite low. It was never designed to do that, it's expected that outages will occur and that's inevitable when you've got lots of moving parts, high pressure, high temperature and so on.
How much to deduct for plant outages is somewhat subjective, it depends how much you want to spend on extra plant versus taking the risk of running short of capacity, but in terms of the Summer rating:
*The original design basis for the Victorian generating plants in question gives an answer of 7795 MW that should actually work on the day. Anything beyond that is a bonus but don't count on it.
*A very common generic approach, which does not take into account the Victorian facilities specifically but is generic, gives an answer of 7887 MW. That is in practice the total installed capacity less heat-related reductions and less the two largest generating units.
So both give an answer of about 7800 MW as the "firm" capacity. In practical terms, that's not far from past experience - two major generating unit outages will give that result and so will a combination of smaller issues that has certainly happened in the past.
Fourth problem is that to be blunt there's some rather ancient and worn out plant in the mix. There's 700 MW of steam plant from 1973 - 75 that's well past its intended design life and not particularly reliable these days. It does work, half of it's running right now, but its presence does pose a risk. Better off with it than without it, but it would be better to have something that wasn't old and tired.
So if we do the sums again then it's 7795 MW generation in Victoria that ought to be reliable.
Plus 478 MW from Tasmania
Plus 200 MW from NSW
Plus 75 MW peaking power from batteries
That comes to 8548 MW versus historic maximum demand of 10,490 MW and maximum in the past 12 months of 9618 MW. Now you see where the concern arises - that's a substantial gap.
So how can it be done:
*High winds and/or mild temperatures in SA will add 700 MW supply from that state.
*Wind generation in Victoria could be anywhere between virtually zero and 1900 MW and is beyond anyone's control. Nature controls whether it blows on the day or not - past history says sometimes yes, sometimes no.
*If the peak's earlier in the day than normal then add up to 285 MW from large scale solar. Typically the peak is late afternoon but peaks during the middle of the day have occurred in the past so it can happen.
*Generating plant working better than expected, with fewer outages, adds up to 1152 MW above the "firm" case scenario. That one's down largely to luck - no amount of maintenance will guarantee nothing fails, especially not when it's being pushed to the limit, but with a lot of luck it all does hold up. Past precedent says it might happen but don't count on it.
End result = it might work but no guarantees. If it's windy across SA and Vic and plant works better than normal then nothing to worry about. If no wind and there's a couple of generating plant outages then all hell breaks loose. A scenario somewhere in the middle may see a bit of load shedding but nothing too drastic.
Those taking an "engineering" view will see that as an unacceptable risk. They'll consider that it shouldn't be down to chance and that the system should be able to meet load with reasonably expected plant outages (Eg two generating units out of service) and under the full range of weather conditions which have previously been experienced. That is, a simultaneous heatwave in SA + Vic with the peak late in the day and minimal wind - a scenario which has indeed occurred on previous occasions.
Those taking a "financial" view will be looking at the cost of building and maintaining more plant versus the hours per year when it would actually be needed and conclude that doing so doesn't really make sense hence why they haven't already built it.
Those taking a "political" view from the government side will note that the first 500 MW of load shedding can be effectively hidden from view by doing it at the Alcoa aluminium smelter and on that basis they can probably withstand the political pain of the reality that even if every house in the state loses power spread over a few months, it won't all be at once and any individual consumer will only be in the dark once or twice over Summer. They'll be ready with some "good news" announcement about funding fire brigades or schools or something to deflect attention from the blackout.
Those taking a "self interest" view will do just that. If they're a business which is plunged into darkness and loses custom then they'll be screaming extremely loudly. In contrast someone selling backup batteries won't be complaining even slightly, indeed they'd like a few more failures thanks.
To the electric vehicles which prompted this comment however, well there's really no reason for concern at all. Looking at data for 31 January 2020 when the temperature reached 42.9 in Melbourne, electricity demand at (times are Vic Daylight Savings time and note these figures do not include the impact of small scale solar generation on house roofs etc):
12:00 = 6062 MW
13:00 = 6821 MW
14:00 = 7302 MW
15:00 = 7798 MW
16:00 = 8441 MW
17:00 = 8921 MW
17:30 = 9195 MW
18:00 = 9352 MW
18:30 = 9330 MW
19:00 = 9420 MW
19:05 = 9453 MW
19:10 = 9364 MW
19:30 = 9191 MW
20:00 = 9027 MW
21:00 = 8444 MW
22:00 = 7488 MW
23:00 = 6447 MW
00:00 = 5998 MW
So
even on an extreme hot day, the period of high demand during which EV charging would be a significant issue is relatively short.
For the record, for that peak at 19:05
Wind and large solar within Victoria = 336 MW
Coal, gas and hydro within Victoria = 8548 MW
From NSW = 487 MW
From Tasmania = 464 MW
From SA = -361 MW (that is, flow was from Victoria into SA)
Figures don't quite add up due to transmission losses not shown. They're a few MW here and there between states.
That there was power from from Vic to SA at that time was primarily for economic reasons. Technically there was about 560 MW available in SA but economics didn't favour it at the time (much of that is diesel plant).
So that's a real example for a day when Vic was very close to the limit, the weather was properly hot, but there was some wind generation running and coal+gas+hydro combined worked a bit better than should be expected and no major drama resulted.
Even on that day, just two hours after the peak demand had dropped ~1000 MW, after 3 hours it was down ~2000 MW and after 4 hours it was down ~3000 MW from the peak. So there's plenty of opportunity to charge EV's overnight even on an extreme day when supply was very tight at the peak.
Based on data extracted from AEMO archives which are accessible at the link below (but it's not in layman's terms, it's the raw data).
http://www.nemweb.com.au/REPORTS/ARCHIVE/
For the record, as of right now demand in Victoria is puttering along at about 4500 MW. There's about 3500 MW of unused generating plant within the state that could run right now and if we take the total across the National Electricity Market states (all states except WA and NT plus remote towns etc in other states):
Available capacity (what's actually available and able to run, so not including anything that's dismantled for maintenance etc): 36,496 MW
Current production: 18,130 MW
So just under 50% of available supply is being used right now with 18,366 MW spare. If the wind stopped completely in every state that would still be about 14,650 MW spare.
That's all pretty normal given it's the middle of the night and that's the point. More use can be made of what's already there if it can be used when demand is otherwise low in order to charge EV batteries or indeed for any worthwhile purpose. Peaks are a problem, we're on a knife edge in some states and load shedding is a real possibility over summer around 6 - 7pm, but middle of the night no worries.
For those who disagree, the data's all available to anyone with an internet connection and spreadsheet software so let's see your analysis.