Cheap storage is more important than conversion ratio. Enough renewables leads to periods of negative prices without matching storage capacity. Storage can mean 1-2c/kwh charging costs, and even 50% efficiency makes discharged power 2-4c/kwh.

if 0.5m thick sphere, 30m diameter is 1413 m^3 of concrete. $300k to $400k in materials. Stores 150mwh power. About $2-$3/kwh