Controlling millions of individual batteries in real-time across the country would be a logistical nightmare. Not to mention wind turbines and solar panels are useless when the wind isn’t blowing at night. You need massive energy reserves somewhere, and not many nations can pull that off with hydroelectricity alone. Batteries or hydro to buffer the sub-10 second demand, hydro or geothermal or gas to buffer the sub-10 minute demand, and nuclear or gas to buffer the longer times.