It’s a great question.
Why is there load shedding at night?
Eskom has a generation capacity of roughly 45GW in total.
Due to decades of neglect, horrifically bad decision making, and outright theft in some cases, this generation capacity is now closer to 30GW; sometimes even less.
What does generation capacity have to do with anything though?
This is easily answered by looking at demand. Sigh, I hear you say. What’s demand?
Demand is the counterpart to capacity. Demand is the amount of power we need to supply at a given time point.
Luckily this is fairly predictable on a daily weekly or monthly basis, and usually measured for time periods between daytime, evening and night.
We can break demand down into rough time bands;
Daytime (5am – 6pm) – we use anywhere between 20-30GW.
Evening (6-9pm) is where our peak usage lies, loads at this time are usually in the high 28-30GW range, as people come home, start cooking, shower etc.
Late Evening (9pm-12pm) our usage quickly reduces to nighttime levels of usage as usage winds back down.
Night (12pm – 5am) we use far less power – around 20GW an hour, which is currently 2/3rds of available generation. While this is less than daytime by a huge margin, it used to be less than half when our available generation capacity was at 40-45GW.
Interestingly this answers a common “issue” that people continually pose about Solar PV.
Solar doesn’t work at night!! so its useless blah blah blah.
True, Solar famously does not work at night, but by a strange co-incidence much our demand comes in the day when we’re.. awake, so thats really where we need generation. By another hugely strange co-incidence night time demand drops quite a bit when we’re asleep. I know that causation doesn’t imply correlation, but it does make you wonder if there might just be something in that.
It’s almost as if we could use Solar during the day to power things, and not have to run other generation, which would allow us to keep other generation ready for when its needed. Crazy idea, I know.
Now we’ve had a rough look at energy requirements during the day, and Eskom’s generation capacity, why is there load shedding at night?
If we look at a statement from Eskom where they have load shed at night:
You’ll see that they load shed at night mainly to replenish emergency reserves.
Let’s break that down, what does that mean?
What are emergency reserves?
Eskom keeps some storage as a backup in case they need to call on it in an emergency.
This could be things like Diesel or Gas, or it could be water (i.e. in pumped hydro).
Think of this as you having a full can of gas ready to fill a generator in the garage in case you need emergency power.
In Eskom’s case that can is empty; they could be waiting on deliveries for the former (Diesel or Gas), or have used up all the latter (water ready to be used for pumped storage)
If they need to replenish emergency reserves, it usually means that they’ve been running the system far in excess of where its supposed to be run. This is usually done at election time for example, where the ANC government explicitly tells Eskom no load shedding.
This costs the country a huge amount, as Eskom then needs to use diesel or gas to power things.
Diesel costs Eskom somewhere in the region of R3.1-R4.1 /kW.h which means they lose a ton of money when this has to be used, as they sell electricity for around R1 / kW.h in bulk.
Interestingly existing coal is still competitive with new solar builds – coming in at R0.40/kW.h
(Pricing from the energy report below)
Our alleged/theoretical total capacity is 45GW, however with ANC misappropriating maintenance funds for decades, this is down to 25-30GW, sometimes even less.
We can compensate for around 2GW.h or so using pumped storage or diesel/gas generation, but this means we have no margin for the next day (our gas can for our generator is empty).
Assuming we have a serious outage and generation sits at 20GW, but we really need 25GW for the day, Eskom will both load shed, and use backup generation. This leads to having no margin for the next days issues, so while overnight load is lower (typically 10GW.h lower), they may need to use all that excess just to refill pumped storage so it can be used again.
As an analogy –
i.e. if are a lot of breakdowns (or demand exceeds capacity), we need to supplement capacity.
We can run our generator (which helps), but then our gas can is emptied.
This means that if there is an issue tomorrow, we’re in trouble as we now have no gas.
If they didn’t load shed at night in those situations, the next day’s emergency would basically shut down the grid, or force severe amounts of load shedding.
In reality its more involved and complicated, but thats the gist of things.
It’s mostly unusual for that evening load shedding to happen, thankfully, as its a lot more serious a situation than most are aware of.
That said, this coming year I expect it evening load shedding to become a lot more common as our Nuclear generation at Koeberg will be going offline for maintenance.
This will leave a huge hole in our generation capacity, and if the other parts of the system fail, then thats a recipe for more cowbell, uh loadshedding.
The unanswered question I hear from you in the back is – what happens when demand exceeds load, and we have no emergency reserves?
Either unannounced large scale load shedding, or the grid will collapse and need to be restarted.
I’m too lazy to answer in more detail (and I’m not Ted Blom), you can find out more on that subject here –
If you are interested in reading more about Eskom’s emergency reserves, and how these work in South Africa, there is a nice thesis online here which goes into more detail – https://open.uct.ac.za/bitstream/handle/11427/13727/thesis_ebe_2015_van_deventer_a.pdf