Don't forget to subtract the electricity used during the daytime to charge the battery.
Why? They came up with 2500 MWh produced and $80/MWh with no difference in price for electricity used directly and electricity used from the battery. There is also 1200 MWh capacity on the battery storage.
Consider:
2500 MWh - 1200 MWh = 1300 MWh during the day and 1200 MWh in battery storage. If you sell that at $80/MWh, that's $104K during the day and $96K at night from the battery, which adds up to $200K.
If you store only 600 MWh from the battery during the day that's:
2500 MWh - 600 MWh = 1900 MWh during the day which is $152K and then $48K during the night from battery. That adds up to... $200K again.
If you store nothing during the day that's:
2500 MWH - 0 MWh = 2500 MWh during the day, which is $200k and then $0 during the night from battery. That adds up, curiously again, to $200K.
Now, I get that there are more complicated models for this where there are different day and night rates and there are some losses from storing power to battery and using it later versus using it right at once, etc. However this was presented as a back of the envelope calculation with ballpark figures and those factors would pretty much just be a rounding error in such a thing. Otherwise, if we're just looking at a simple model of a set amount of electricity per day and a set price, there is no point in subtracting the electricity used during the day to charge the battery. Just like there is no point in subtracting inventory in a 24 hour store that you keep in a back room and restock around evening and then sell overnight.
There is, of course, overcapacity as a consideration. Do you end up producing more electricity than you can sell? However, that's not really a consideration of the model at this point either. The assumption is that the amount of electricity generated is close enough to the amount sold that the difference is pretty negligible.