[BradyLS]

We start with the current usage that must be supplied. Currently, the usage ranges between a low of around 30 GW and a high of around 40 GW over the course of a day.*For purposes of this exercise, let’s assume an average usage of 35 GW. Multiply by 24, and we find as a rough estimate that the system must supply 840 GWH of electricity per day.

* Surely the author meant "over the course of an hour because they multiply that number by 24 to arrive at the 840 GWH figure for the average daily usage. There was no reason to multiply by 24 if the 35 GW is the average for a day...Or what did I miss?

[BradyLS]

...Unless a GWH is 1/24 of a GW??? That doesn’t seem intuitive to me.

[Chickensoup]

no you are correct.

I caught that too.

[KarlInOhio]

It looks right. The 30-40 GW is the flow rate of energy (aka the power) while multiplying it by time gives you the total energy 840 kWh. The problem is that we are used to flow rates being in amount per time but in this case the units of watts and kilowatt-hours don't match that because the flow rate is the base unit.

If you think of it as 35 miles per hour (the flow rate) multiplied by 24 hours to get 840 miles you can see what is happening.

[Robert A Cook PE]

? 35 was the writer’s (rough) average for each hour of the 24 hour-long day.