Nope they probably mean 55 GW. Right now peak for just CA ISO is somewhere around 50 GW. Daily usage average for the year is around 600 GWH. More in summer less in winter.
As a side note 55GWH would more or less than half the current(2020) average daily solar in summer. So it might fix the duck curve in summer currently. Not the current demand of between 600-900 GWH demand in summer day.
How long must the "Long-duration storage" store energy for?
California's peak demand of 55 GW in 2045 is estimated in the (above) link. To supply 100% renewable energy with that peak demand in sustained bad weather, they estimate that California would need a total "long-duration storage" able to accumulate and supply the maximum load of 55 GW. However the article does not state how long it would have to store it for, or for how many successive days of bad weather it must supply up to 55 GW.
Our ThinksTooMuch helped Roger Andrews calculate (see webpage) that with 55 GW of demand, 17 TWh of PHES would be needed to store energy between good and bad seasons. That is, for months. If there were no losses, that could store the equivalent of 13 days at full blast (enthusiasts for lithium batteries can only claim a few hours) of 55 GW, but alternately trickled out and topped up across the year. However there certainly would be losses such as evaporation from their PHES.
Salt+water is exothermic! Completely reversible to store more heat. Recharge with the waste heat from industrial processes. Ingenious and appears to be almost immediately practicable. Ought to win an award, in my opinion.