1^m3 of 300 bar air = approx. 30 kwhr.
Approximately 1/4th of that in water storage will hold the heat.
We're aiming at a daily loss of 1%, but it caps out at 10% relative (~7% absolute) because the Energy Out/Energy In is proportional to T_exp/T_comp (in an absolute scale) -- if the temperature drops to ambient, T_exp is only slightly lower in an absolute scale.
What's the real energy efficiency of the system?
What's the estimated energy loss per unit time of the storage system?