Is it 300 or 600 MWh? Or is the storage 600, but it can deliver 300MW/h?
Edit: The company press release is much clearer: https://stateraenergy.co.uk/news/thurrock-energisation The storage is 300 MWh, but it can deliver a peak of 600 MW/h (presumably for half an hour).
The OP says it the other way around though. Power is 300MW and capacity 600MWh=2h*300MW. There is no mentioning of 300MWh in the OP.
> This landmark 300MW battery storage site is capable of powering up to 680,000 homes with instantaneous power over two hours
Power is 300MW (300000000 Joules/second), which it can deliver for 2 hours, so capacity (energy contained in the device) is 600 MWh (or 2160000000000 Joules)
The orginal article is pretty clear:
The 300MW Thurrock Storage project... with a total capacity of 600MWh