Yes, "1000 gal/d of storage capacity" is valid. "1000 gal/d" is not. Even if you used 1000 gal/d colloquially, you would have been talking about the capacity of

Yes, "1000 gal/d of storage capacity" is valid. "1000 gal/d" is not. Even if you used 1000 gal/d colloquially, you would have been talking about the capacity of the bottles. The misconception you're having here is that "GWh of storage" is the volume of the bottle. You're interpreting it as "GWh", which in the bottle example would be like saying "They produce 1000 gallons of water / d".