You don't usually need to store energy for a significant period of time, just until the daily peak. Last I checked, the efficiency for charge + discharge was something like 70%, which, in any case, is much better than the 0% energy savings from using that energy to mine crypto.

However bad batteries are at storing energy long-term, I guarantee that bitcoin mining rigs are worse. It's hard for a battery to do worse than losing 100% of "stored" energy immediately to heat.