[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.

[BradyLS]

I can understand that. Then a GW is the unit for a day and a GWH is 1/24 of a GW. At least as far as the math in the article goes.